Hubert Timmenga is a soil scientist and environmental scientist practicing agricultural consulting in Western Canada. He obtained a Master’s degree in Environmental Science from the University of Wageningen in the Netherlands and earned a PhD in Soil Science from the University of British Columbia in Vancouver Canada. He recently completed Geoff Lawton’s 2015 Online PDC Course. Geoff suggested that Hubert write a paper on rehydration to encourage bigger-picture thinking. This paper is the result of that suggestion.
This paper reviews the various concepts related to food security, sustainable agriculture, diversity farming and rehydration (the introduction by design of surface run-off or streambed water in the soil), and explains how these concepts interact and how they may be affected by natural elements such as soil and forest cover. It is concluded that while rehydration will provide ecosystem services such as wetland creation and reduced erosion, by itself it will not lead to sustainable food security. Only when rehydration in an appropriate form is combined with diversity agriculture and sustainable practices will it support food security.
In many areas of the world, water is the limiting factor in agriculture and food production. The lack of water during the growing season can be caused by, but not limited to, a restricted supply (no rain), limited distribution (no opportunities for irrigation), no storage and fast run-off to beyond the target area, lowered aquifer due to stream bed erosion, high evaporation rates in arid and semi arid areas, or lowered aquifer through pumping. This paper will review some of the solutions relating to lack of water including water harvesting, stream bed protection and rehydration of the soil. Any of these solutions will result in increased supply through collection and storage, prevention of run-off, and soil infiltration other than direct irrigation. However, methods to reduce evaporation will not be addressed here. In addition, such systems or combination of systems would support food production in a target area, resulting in food security for the inhabitants. By proper design of a water supply system to be integrated with a farming system, food security could be enhanced through improved sustainability.
This paper will address the various components and approaches based on the premise that supplying water in a sustainable manner (e.g. rehydration) can be a corner stone of sustainable food production and, combined with a diverse agricultural system, can enhance food security. Specifically, this paper is focused on the role of rehydration on small scale agriculture or food production designed to sustain single families and groups of families, with a limited watershed size area rather than a region, province or country. The set-up of the paper includes discussions on food security, diversity farming, sustainability and rehydration, and provides working definitions for each. The resulting discussion will address how rehydration of soils can support sustainable food security.
The World Food Summit of 1996 defined food security as existing “when all people at all times have access to sufficient, safe, nutritious food to maintain a healthy and active life”. Commonly, the concept of food security is defined to include both physical and economic access to food that meets people’s dietary needs as well as their food preferences. As well as having adequate water and sanitation, food should be available in sufficient quantities on a consistent basis, persons must have sufficient resources needed to obtain appropriate foods for a nutritious diet and food should be used based on knowledge of basic nutrition and care.
Food security, a flexible concept with many complexities, triggered discussions that have resulted in the following definition: “Food security [is] a situation that exists when all people, at all times, have physical, social and economic access to sufficient, safe and nutritious food that meets their dietary needs and food preferences for an active and healthy life” with a working definition as: “ Food security exists when all people, at all times, have physical, social and economic access to sufficient, safe and nutritious food which meets their dietary needs and food preferences for an active and healthy life. Household food security is the application of this concept to the family level, with individuals within households as the focus of concern”.
This working definition will be used to address food security for families or groups of families.
Farm Diversification and Diversified Farming
Farm diversification is a world-wide movement created to branch out from traditional farming by adding new money generating activities. The concept is widely advocated in conventional farming in Europe, North America and other areas and is supported by international organizations such as the World Bank. Examples include the rice-fish co-culture in China with lower requirements for pesticides (68% less) and chemical fertilizer (24% less), and the rice – banana – dairy system in India, which was found to be more sustainable due to the financial and market incentives of the demand for milk products.
Herrero et.al. (2010) discusses that sustainable food production is enhanced by a mixed agricultural system, where livestock provides nutritional input to crop land, and generates alternative income streams. While such highly intensive systems may still affect the environment, effects can be prevented by less intensive methods. These science-assisted methods include appropriate land preparation, timing of planting, use of fertilizers and pesticides and modified crops. Further, investment should be made to pay for protection of water, carbon, biodiversity and ecosystem services to de-intensify or cease the implementation of industrial, monoculturally-based conventional agricultural systems. Farm diversification may also include ecological services, and agro-tourism.
Diversified farming is a management technique where farmers diversify their crops by including different commodities and combining livestock with field crop production, and/or vary their marketing systems. It is farmer or product oriented and monetary gain or farm survival is an important driver. Diversified farming should not be confused with biodiversity in farming which is focused on the support (eco) system for a farming operation. Biodiversity in farming is dealing with the survival and increase of non-marketable plants and animals in an agro-ecosystem and is commonly interpreted as species richness, and occasionally as the richness of varieties, cultivars or genetic expressions. Rahmann, (2011), found in a large meta study of European studies (766 papers) that more than ¾ of the studies showed higher biodiversity in organic farming, 14% showed no difference and in 3% organic farming yielded less biodiversity. Pfiffner and Balmer, (2011) show in a meta study that organic farms, i.e. growing crops without pesticides or chemical fertilizer, have between 46 and 72% more semi-natural habitats and host 30% more species and 50% more individuals than non-organic farms, providing farmers with the benefits from an intact and sustainable functioning ecosystem.
Regenerative organic agriculture as tested for over 30 years by Rodale Institute on their farm, is a system that provides legume based and organics based nutrition and incorporated no-till systems. Rodale has proven that such a system has yields equal to or better than conventional production, especially in drought years, while being more profitable, using 45% less energy, and producing half of the greenhouse gases. Crop rotations made this organic system more diverse. It was found that organic systems improved soil health, increased ground water recharge and reduced run-off. Rodale’s organic farming system is considered sustainable. Other work by Cong et. al 2014, showed similar results in mixed species grassland without legumes, the soil C stock and Soil N stock were respectively 18 %and 16% higher than in monocultures, and that in rotational strip intercrop systems similar results were found as compared to ordinary crop rotations. Strip intercrop systems increase yield, better soil quality and soil C sequestration (Cong et.al., 2015 ).
Agro-ecological farming practices focus on small scale agriculture with an holistic approach to the functioning of farms and with application of ecology to the design and management of sustainable agro-ecosystems and based on food system development originating from traditional knowledge, alternative agriculture and local experience. Such systems: maintain their natural resource base; rely on minimum artificial inputs from outside the farm system; manage pests and diseases through internal regulating mechanisms and, recover from the disturbances caused by cultivation and harvest.
Biodiversity in farming and diversity in farming have been combined in the concept of Diversified Farming Systems (DFS), which was introduced in a series of papers in 2012 (Kremer et. al. 2012 ). It is “as farming practices and landscapes that intentionally include functional biodiversity at multiple spatial and or temporal scales in order to maintain ecosystem services that provide critical inputs to agriculture such as soil fertility pest and disease control, water use efficiency and pollination”. DFS may include multiple genetic varieties, and/or multiple crops grown together as poly-cultures and also may include the stimulation of soil biology through addition of compost or manure, all following a holistic approach.
DFS depends on many cultures, practices and governance structures to support management practices adapted to the local environment, often based on traditional agriculture. DFS is not really focusing on the water management aspects of agriculture and food production but was developed more from the point of view of supporting ecological goods and services such as soil fertility and pollination. Ranching in the US and that country’s diversified practices was cited as part of DFS to include multi species grazing, matching livestock to forage conditions, use of conservation easements, diverse products and alternative marketing systems. As well, agro-tourism and other services were included. In this respect DFS ranching is not much different from the diversified agricultural practices that are advocated in Canadian Prairie Provinces such as Alberta and Saskatchewan and in parts of US conventional agriculture.
DFS appears to be very similar to Permaculture, a sustainable food producing system first described in the 1970s and 1980s by Mollison in Australia and now promoted world-wide through efforts of others including the Permaculture Research Institute (PRI). Permaculture is the conscious design and maintenance of agriculturally productive ecosystems which have the diversity, stability, and resilience of natural ecosystems and is the harmonious integration of landscape and people, providing food, energy, shelter, and other material and non-material needs in a sustainable way.
Permaculture design is a system of assembling conceptual, material, and strategic components in a pattern which functions to benefit life in all its forms. It includes ethics and guidance such as conservation of natural forests, rehabilitation of damaged natural systems, establishing plant communities and systems on the least amount of land while integrating ecological services to protect rare or threatened species. Permaculture includes water harvesting, water storage combined with active and non-active water supply methods as integral parts of system designs.
Farm diversification appears to be geared towards farm profitability and survivability as an economic unit through inclusion of other (related) market opportunities in the management plan, such as different varieties of the current crop or livestock, adding different crops or livestock, providing ecological services and advancing various marketing opportunities. Diversified farming and Permaculture would include all of the above but is focused on a holistic approach towards ecosystem restoration and full integration of various crops, forages, livestock, ecological services and marketing. For the purpose of this paper, diversified farming or diversity farming is defined as:
Sustainability or Sustainable Development has been discussed for many years. One of the important documents in which these concepts were presented is the Brundtland Report of 1987 (Our common Future). The report includes a definition for sustainable development as “development that meets the needs of the present without compromising the ability of future generations to meet their own needs”.
Definitions of sustainability have evolved since that time. For example the following definitions have been presented:
a : of, relating to, or being a method of harvesting or using a resource so that the resource is not depleted or permanently damaged
b : of or relating to a lifestyle involving the use of sustainable methods
Definitions for sustainability are quite open ended and raise the question of how to determine whether a process, product or facility is “sustainable”. Methods were developed to address this determination, including the Triple Bottom Line (TBL) approach and others such as the 3P concept. The Brundtland concepts, TBL and 3P concepts have been modified/criticized over the years. For example Seghezzo (2009) proposes a variant on the 3P concept as Place, Permanence and Persons, where the Place is the 3 dimensional location or area to be considered, Permanence the effect of time, which in the view of Seghezzo has largely been neglected in the sustainability debates in spite of the widespread recognition of the potential long term effects of actions, and the Person as an individual rather than undifferentiated members of society.
The triple bottom line (TBL) is an accounting framework that incorporates three dimensions of performance of organizations including social, environmental and financial, rather than financial only. TBL is sometimes acknowledged as three Ps (3P): People, Planet and Profits (or Prosperity). TBL captures the essence of sustainability by measuring the impact of an organization’s activities on the world, including its profitability and shareholder values and its social, human and environmental capital. Measuring of the various parameters making up TBL is difficult and can be cumbersome.
Metrics for parameters can be used to view a trend in an organization or to compare the performance of organizations or processes. It should be noted that various types of organizations, processes or products each may have a different set of metrics. The TBL accounting has been adapted in a set of guidelines for the accounting of long-term sustainable performance that relies on an understanding of the interdependency between financial, social and environmental factors. Such guidelines are combined in an accounting system that specifically involve sustainability and target financial decision makers in companies known as the A4S CFO Leadership Network, which is supported by Prince Charles of the UK.
In the hierarchy of concepts it appears that there are different levels or connections for which sustainability is being used as a measure: in time line – this year is more sustainable than last year in our corporation; in comparison – this product, institution, government program, corporation, is more sustainable ; and in absolute terms, this is sustainable food production. Comparisons, both in time line and product/institution depend on metrics, which should include similar parameters. However, for each comparison a new or different set of metrics is developed. Product-specific standards and overall standards have been developed for a variety of products. For example, he Sustainable Products Corporation in Washington DC, includes several of such standards in their documentation including those for wood, clean vehicles, certified organic products, ePower, LEED rating for buildings, Salmon friendly Products, Cleaner and Greener Certification and also specific logo/labels such as Natural Step Systems, Nordic Swan EcoLabel and Green Seal Products. The company works on the Life Cycle Analysis principle and metrics and defines sustainable products as “ those products providing environmental, social and economic benefits while protecting public health, welfare and environment over their full commercial cycle, from the extraction of raw materials to final disposition.”
Using metrics to determine agricultural sustainability is still in its infancy. For instance in March 2015 it was announced that the Centre for Agricultural informatics and Sustainability Metrics is to be established at Rothhamsted Research in the UK with cooperation of a consortium including Rothamsted Research, University of Reading, the National Institute of Agricultural Botany and Scotland’s Rural College. This decision will position the UK as the leader in the field.
Hobbs (2006) advocates conservation agriculture (CA) for sustainable production. CA is defined as a system with minimal soil disturbance (no-till) and permanent soil cover (mulch) combined with rotations. Sustainability is achieved through higher yield, lower costs of inputs and time, increased water use efficiency, better soil quality, stronger plants, and less weed pressure with higher biodiversity. Parr et al (1990) defines the concept of low-input/Sustainable Agriculture as follows:
“Low input farming systems seek to minimize the use of eternal production inputs (i.e. off-farm resources) such as purchased fertilizers and pesticides wherever and whenever feasible and practicable, to lower production costs, to avoid pollution of surface and groundwater, to reduce pesticide residues in food, to reduce a farmer’s overall risk and to increase both short and long term farm profitability. (…)” through good management of on-farm resources and crop rotations to provide the necessary levels of plant nutrients and to conserve available soil moisture.
Grace Communications Foundations defines sustainable agriculture as:
“the production of food, fiber, or other plant or animal products using farming techniques that protect the environment, public health, human communities, and animal welfare.(…) without relying on toxic chemical pesticides, synthetic fertilizers, genetically modified seeds, or practices that degrade soil, water, or other natural resources. By growing a variety of plants and using techniques such as crop rotation, conservation tillage, and pasture-based livestock husbandry, sustainable farms protect biodiversity and foster the development and maintenance of healthy ecosystems.”
The agroecology definition of sustainable agriculture is:
“A whole-systems approach to food, feed, and fiber production that balances environmental soundness, social equity, and economic viability among all sectors of the public, including international and intergenerational peoples. Inherent in this definition is the idea that sustainability must be extended not only globally but indefinitely in time, and to all living organisms including humans”.
Many jurisdictions have described desirable sustainable food systems. This appears more related to the consumer rather than the producer of food, and also includes processing and transportation of food in addition to its taste and energy requirements (Province of Alberta, City of Calgary, University of Chicago and others).
The European Union defines sustainable food as follows:
“(Sustainable food) implies the use of resources at rates that do not exceed the capacity of the Earth to replace them. For food, a sustainable system might be seen as encompassing a range of issues such as security of the supply of food, health, safety, affordability, quality, a strong food industry in terms of jobs and growth and, at the same time, environmental sustainability, in terms of issues such as climate change, biodiversity, water and soil quality.”
Community Research Connections, (Canada) in a report from 2007, indicates sustainability in food production related to farmers markets as follows:
“Sustainability refers to following food system practices that respect the ability of future generations to meet their food requirements. This includes environmental protection, profitability, ethical treatment of food system workers and other living beings, and community development. Local food systems meet these criterion to a much greater extent than industrial food systems, due in part to having to deal with direct consequences of food system decisions, ” …[local food systems] “are rooted in particular places, aim to be economically viable for farmers and consumers, use ecologically sound production and distribution practices, and enhance social equity and democracy for all members of the community;” and “tailoring food production and its consumption to local conditions is believed to be a key factor in developing sustainable food systems.”
Some definitions for sustainable development and sustainability include the term or the concept of permanence. From the above discussion it is understood that permanence is related to long term survival of a system, to provide services, or products over an indefinite period. In this light, one should include that sustainability not only means to stay with the status quo, but also improve upon it. This is a concept used in various industries in order to improve quality or sustainability through “continual improvement”. Permanence can be explained differently, however. For instance Jacke (2005) sees permanence in systems as the elements that cannot be changed (or with great difficulty or at great costs) such as climate, land form, water and supply, and legal issues. For this paper, the former interpretation of permanence will be used.
The working definition for sustainable food supply in the context of the current discussion is;
Historically, water was collected from rainfall- as well as water collected from streams and dug wells – was stored for domestic use as drinking water. Currently in the developed world water is supplied through on-site wells and municipal systems. Water harvesting may thus be increasing supply and reducing the pressure on both the on-site and municipal system. Supporting reasons for water harvesting include:
1. Provide water for domestic/livestock purposes and prevent costly upgrades of municipal systems for water supply. Water is collected from roofs or from land surfaces and stored for domestic use. Tanks are used and water is cleaned by filtering and disinfection. Water is then used for toilets etc. in homes to save up to 85% of the total water supply. Farm dugouts on the Canadian Prairies have been used as water storage reservoirs for many years. Due to unavailable ground water or ground water with questionable quality, impounding run-off is often the only means of ensuring continuous supply of farm and domestic water. In the 1930s the government supported the establishment of surface runoff collection systems. Typically, water is harvested via natural drainage, but the use of contour swales to collect water is seen. Water quality is managed through aeration, cleaning and where needed, the use of algaecides such as copper sulfate.
2. Provide water for irrigation purposes Water for irrigation of gardens can also be collected from roof run-off and stored in tanks directly fed by down spouts. Water is typically not treated for garden use but a first flush diverter or a filter may be in place. On a larger scale, water can be collected by diverting overland flow through channels to ponds for storage. This water is then used when required in agricultural crop production and livestock watering through pumping or hydrostatic systems.
3. Prevent surface run-off and erosion by catching water flows. Water harvesting is implemented in such areas to reduce or eliminate the use of water from the municipal supply for irrigation of ornamentals. Precipitation run-off is collected in wells around trees and shrubs, run-off from hard surfaces is collected in concave lawn areas and planter beds for soil infiltration rather than surface run-off, roof run-off is collected in bermed landscape holding areas or French drains for infiltration, or is collected and stored in barrels, tanks or underground storage for later use and irrigation. Stored water may be filtered and disinfected before use.
4. Prevent costly upgrades for municipal drainage systems and water treatment facilities that are receiving water from storm run-off. Various cities including the City of Vancouver in Canada, have changed storm water collection via a (combined} sewer system into on-site water infiltration systems. Impervious surfaces are minimized and street run-off is collected in plant beds and then infiltrated into soil via swales and rock pits. Parking spaces were covered with Golpa, a weight supporting open plastic matting filled with top soil to support grass.
IFAD – International Fund for Agricultural Development stated that water retention is the key to livelihood security in low rainfall areas. Periodic rainfall needs to be conserved through safeguarding of natural storage and recharge functions, maximizing the potential of large public infrastructure to contribute positively to local water conservation and recharge, and the implementation of a wide range of water harvesting measures. Surface-water harvesting allows collection of water in reservoir tanks and cisterns from roof tops and ground catchment systems. Excess water is stored in shallow aquifers. Run-off can be intercepted through contour trenches, spread through infiltration ponds, tanks, inundation canals of flood irrigation and recharged through river banks and the modification of river channels. IFAD promotes investment in water harvesting methods (in particular, traditional methods), the development of large scale measures and in capacity building.
While in some areas of the world great strides are being made towards water harvesting the technique is not always high on the agenda of organizations, or is difficult to implement. Rockstrom and Falkenmark’s paper on rainwater harvesting in Africa (2015) notes that water issues are not included in the UN Sustainable Development Guidelines while they are under development. The paper indicates that in arid and savannah systems (in Africa) the green water (i.e. moisture from rain held in the soil) is an important source of moisture for crop growth. As 50% to 70% of rainwater evaporates, it generates very little blue water or run-off – meaning little of it recharges rivers, lakes or ground water. Better collection and storage of rain water would tide farmers over during unexpected dry periods in the growing season. Green water practices such as soil and water conservation and water harvesting are increasing in areas where irrigation with blue water is impractical. Improvements in green water harvesting would improve local food production as seen in Kenya, Tanzania, India, China and other areas. Green water management would benefit from conservation tillage, mulches and canopy cover. The authors advocate nutrient additions to boost productivity further.
The InterAmerican Institute for Agricultural Cooperation (IICA) outlined the potential improvements to water use and water efficiency in Latin America in order to reduce the pressure on water resources, reduce environmental degradation and improve food security by combining scientific knowledge and traditional and ancestral knowledge of farmers. IICA sees improvements in plant water use, as well as in better water use on the farm, including irrigation techniques, protected agriculture and hydroponics. Conveyance and distribution of water is addressed through better operations of systems, by reducing water evaporation, reducing filtration, run-off and leaching, minimizing water pollution and salinization, and promoting the recycling of water. Palmier (2003) found that recent water harvesting projects have not achieved their expected goal as the technologies and designs were not suitable for either the environment or the cultural habits of the beneficiaries and were found unsustainable. In addition, operation and maintenance of the schemes turned out to be either too costly and/or time consuming. In his final conclusion, Palmier stated that soil and water conservation which can be achieved by using water harvesting techniques represent part of the basic infrastructure for sustainable agriculture.
Rehydration of soils or landscapes is a concept in which water is managed to increase the moisture levels in soil for beneficial reuse. The term soil rehydration is not often used in relevant literature and is related to work done in the restoration of large wetland complexes including those in Europe (Danube Delta) and South East Asia (Mekong Delta). Wetland restoration may contribute to benefits such as improved water quality, carbon storage and biodiversity, all resulting from a general rehydration of the landscape as indicated by Maltby and Barker (2009). Widows (2015) commented on regular water management and broadened the new paradigm description to include the small and large cycles of water as well as water conservation taking place in forest cover and increased carbon content of the soil as described earlier by Kravcik et. Al (2009).50
Soil rehydration can be as simple as raising a water table in a flatland field through the increased water levels in ditches that surround it, or through the increase of organic matter in the soil that facilitates soil infiltration rather than surface run-off as described in long term experiments at Rodale Farms.
Soil rehydration is also used in flood plain areas for restoration. In this instance, wetlands have become unplugged by guttering and gully erosion, riverbeds have formed deep water channels that leave the floodplains dry and, combined with over grazing, the ecological system will deteriorate. Bioretention was derived as a system to re-hydrate the landscape based on management of stream flows in eroded creek beds in grassed alluvial plains in Australia. It includes creating “leaky weirs” from locally available materials such as bundles of wood or rock gabions to slow down the water in the creek beds and trees with fibrous roots such as willows are planted to stabilize the stream banks and the weirs. The role of weirs include: backing sediments that raise the creek bed rather than having the sediments move with the water and creating infiltration ponds from which water is infiltrated into surrounding wetlands. From there, the aquifer is replenished to its former level in the flood plain rather than having water channeled out of the area.
Such work has been documented by Steeton et.al 2013., and others have described implemented projects based on this concept, with several case studies published. Similar work has been described by Tinley and Pringle (2013, 2014) in projects designed to curtail erosion patterns in large Australian landscapes responsible for causing changes in soils, plants cover, fauna and the demise of wetlands and flood plain grasslands. Because erosion causes widespread and continuous depletion of soil moisture, rehydration by rain water harvesting is seen as a key factor in management requirements to revive bottom pasture lands for grazing stock. It is noted that grazing pressure needs also must be managed. The concepts of Tingley and Pringle have been adopted in other areas; for instance, The Southern African Science Service Centre for Climate Change and Adaptive Land Management (SASSCAL) is now training land holders, stakeholders and decision makers in the region to repair leaky landscapes.
Other infiltration systems are more specifically designed for crop production rather than wetland restoration and grassland rehabilitation. The Yeomans Keyline design first published in 1954 continues to be used in Australia, Europe, North America and elsewhere. Keyline is a comprehensive whole farm water management plan that uses natural contours and specific cultivation techniques to harvest rain water. This plan is based upon establishing irrigation ponds on specific places in the landscape with graded earth channels to harvest and store run-off water and to use fast flood irrigation of flat productive lands. Pasture renovation with chisel plows and contour plowing is part of the Keyline design; trees may be incorporated in the designs. Other systems are largely based on the Keyline principles.
For example, The Permaculture Research Institute (PRI) in Australia, and similar institutes elsewhere, focus on water harvesting and rehydration of soils in hilly areas of the subtropical/Mediterranean climate zone and other areas. Principles of their design are based on the work of Mollison (1988). Their concept is based on building contour capture swales that are used: to infiltrate water, to capture water in ponds for beneficial reuse in e.g. irrigation of productive fields down slope, and to utilize the pond and the swales as a surge system to reduce peak water flows to match soil infiltration capacity. Captured water is also directed to soil mounds directly down slope of infiltration swales for slow soil infiltration. Any excess water beyond the infiltration capacity of the system is directed to natural stream beds via overflows and spill ways. PRI re-vegetates the infiltration soil mounds with productive fruit tree species and shrubs and nitrogen fixers to improve productivity and greatly reduce maintenance efforts. Land between swales is destined for food crop production or for grazing by livestock.
For the purpose of this paper rehydration is defined as:
In this definition, permanence is included as rehydration should be set up based on landscape modification such as building swales, placing earthwork structures in creek beds etc. that will serve their purpose in perpetuity. Earthworks should be designed to be maintenance free in the long run. This means, for instance, when placing weirs, gabions or other flow-slowing earthworks in creek beds these should become part of the landscape. It also means that collected sediment originating from other sources than the streambed itself should be dealt with. Non-mechanical means are included in order to focus on natural processes and not on pumps and other equipment that require outside energy and maintenance. The definition would cover river modification programs and wetland restoration projects designed to store more water and reduce flow.
Soil management is an important factor in any form of (sustainable) agriculture. For example, soils have important functions in water management and primary production including storing carbon and nutrients for slow release to plants. Sometimes “soil” is interpreted widely but in the strict soil science interpretation, it is the layer consisting of organic and mineral elements that has been affected by soil forming factors. The soil consists of layers of organic matter on top (L, F, H), a layer that is enriched with organic matter (Ah), layers that have been leached and lost clay, minerals and nutrients, (Ae) and then layers that have been enriched or otherwise affected (B horizons). Unaffected parent material forms the C horizon. Horizons can be of different textures, organic matter levels etc. The soil is measured between the surface and the top of the C horizon. This sequence of horizons is called a soil profile and they can be categorized in 32 Reference Soil Groups that are separated in 8 groups based on specific characteristics (FAO 2014) . Plants typically root in the A horizon and the B horizon, however water that transfers to the lower soil may be out of reach of plant roots.
“Soil” as defined, is at least 10cm deep and can be as deep as 2 -3 m in humid areas; its depth depends on the time and the action of soil forming processes. Soil in old tropical ecosystems can be very deep as the soil formation processes, including climate, rainfall, and plant growth, have had a long time to convert parent material. It should be noted, though, that some consider them shallow as the organic layer is thin and has a very high turn-over rate due to biological breakdown activity stimulated by high temperature and high moisture. The majority of roots in a tropical system are in this organic layer, but some tap roots can be as deep as 60m. Root penetration in the soil is restricted by the presence of a high water table or any constrictions such as cemented layers or bedrock. Except for the deep tap roots in tropical soils and of mesquite trees in some arid regions, roots in temperate systems are generally found in the top two meters of the profile with 50% in the top 0.3m. Fine roots, fungal mycelia and decomposed organic matter form stable soil aggregates .
Forest covers would affect the local climate and reduce the runoff of water. The sections below will address how forest cover effect soil hydrology. In dealing with this topic the comprehensive work by Chang (2012) has been sourced.
Forests cover many areas of the world with forest growth depending on the temperature and moisture content of the soil. While trees are dormant over the cold period, the growing season starts with mean temperatures of about 6oC and may be regulated by heat units. If enough heat units are reached for a specific species, the winter dormancy is broken. Typically, forests need at least the equivalent of 300 – 400mm of annual rain to grow. This can be from precipitation, fog condensation or irrigation. Trees stop growing when under drought stress and in many areas they go dormant during a summer drought. In cold temperate climate one may see two dormancy periods, a long winter one and a short summer period when the soil lacks water. Mature natural forests are diverse ecosystems in which precipitation is absorbed in the organic rich top soil for plant growth and evapotranspiration. Surplus water flows downhill through the soil until it is intercepted by creeks or gullies and then flows to outside the soil system. Water leaves the system as overland flow when the rainfall intensity is higher than the absorption capacity of the soil. The former flow has been typified as green water and the latter as blue water by Rockstrom and Falkenmark (2015).
When a forest receives rainfall, about 10-25% is lost by canopy interception and is directly evaporated. Junipers in Texas for example, evaporated all the received rainfall from a 6.4mm storm and lost 48% of their annual rainfall in direct evaporation. Rainfall over the level of canopy interception is called throughfall and stem flow. This water can reach the litter layer where it is absorbed before water reaches the mineral soil. This total forest interception can be modeled based on stand age, the number of storms and the total rainfall of the area. Forest interception is not available for recharge of ground water or for biological processes beyond the litter layer.
Water can be obtained in low precipitation areas from dew drop or tree drip. During the night dew is formed on the leaves or needles when the dew point is reached. Tree drip typically occurs in Mediterranean climates near the ocean or at higher elevation and is reported from the West Coast of the US, Hawaii, Venezuela, Colombia, Panama, Spain, Chile, South Africa, Oman and parts of England. Other places with similar conditions may also have some dew drip. Fog drip provides an increase of net precipitation in forested system compared to non-forested areas and can contribute up to 100% of the water required by certain forests.
Water loss in vegetation is through potential evaporation (PE). Several methods exist for calculation of PE based on sun radiation, day length, temperature and others. Generally PE measurements are available for many jurisdictions. PE ranges from 1.8 to 3.2mm/day in forests, and in high temperature dry areas can be greater, including ecosystems such as citrus plantations, grass or open water. However, the decrease in moisture content in the soil in a forest is faster than that of a non-forested area because of the great transpiration area of trees, the deep root system, canopy interception losses, more available energy and wind effects. However, trees can absorb water from greater depth and keep active even if the soil dries out at the surface. More than half of the water in a forest is lost through transpiration, about 30% through canopy interception and the remainder through soil evaporation. Transpiration is highly species dependent and negatively correlated to humidity in the air.
Water not absorbed in the plants or evaporated (both canopy and evapotranspiration) travels to a stream in three different ways: overland run-off, subsurface runoff and ground water runoff. Quantities to the stream also depend on other factors such as internal storage of water in the system, while soil infiltration and flow-through depends on soil conditions such as texture, cover roughness, temperature and soil moisture. Infiltration in a forest can be many times greater than on bare ground. Coarser soils infiltrate faster but hold less water than finer textured soils. Infiltration rates are lowest for wet, swelling clay soil and highest for deep sands, loess/aeolian soils or aggregated silts.
Subsurface flow moderates the flow to a water course while surface run-off provides quick feeding of the stream. Due to the differences in rates of soil infiltration, overland flow in rangeland and crop land is much higher than for forests. Forests typically have most of their runoff as subsurface runoff. Surface runoff will only take place if the rainfall intensity is beyond the infiltration capacity of the soil, e.g. in heavy storms or with prolonged rainfall. The difference in surface flow between a forest (quite low) and rangeland (high) affects the amount of water that is directed to a water course. When forest is removed more water moves more quickly and causes peak events in the water course.
After the forest is removed through harvesting, it is often replaced by grazing lands or cropland. Through over-grazing and over-cropping the natural nutrient storage is reduced and productivity is lowered over time. Such systems do not have the water storage capacity of the diverse mature forests they replace, restricting plant growth through drought stress. Lower water capacity also results in a greatly increased surface run-off, leading to soil erosion, clogged drainages, flash-floods, debris flows and mudslides. In agricultural systems, soil conservation methods are used to decrease soil erosion due to surface run-off. Methods used include contour plowing to rough up the surface, strip cropping to limit the slope length, and installing grass strips and vegetated drainages to catch soil run-off. Such methods can reduce the movement of soil but may not have large impact on the surface run-off of water.
Evaluation and Conclusion
Although it will provide large benefits to ecosystems and biodiversity, rehydration by itself in any form will not provide for food security, diverse agriculture or sustainable production. Rehydration will support biological systems that provide ecosystem services such as improved or restored wetlands, agricultural systems such as grazing lands and, through added irrigation works, crop lands and soil support systems such as forests. With good design and implementation, rehydration efforts can be sustainable in light of the permanence requirements.
In order to obtain food security for a family, group of families, or a region where rehydration is implemented, food producing systems must be incorporated in the design and implementation of rehydration measures. For food security to be effective, food production should be diverse in marketable agricultural output and management systems and include annual and perennial vegetables, including fruit-bearing trees and shrubs in addition to various forms of livestock.
A diverse agriculture must include all the management techniques that have been found to increase diversity and sustainability including organic agriculture, parallel cropping, polyculture, forest cover for soil stabilization, production of fruits, fuels and building materials, conservation agriculture, conservation tillage, water harvesting and irrigation, rehydration, various marketing techniques and supporting land tenure and social systems.
Sustainable agriculture is a permanent system of food production without non-renewable inputs. It must be considered that a sustainable agricultural system improves the livelihood of its participants rather than maintaining a (depressed) status quo. Food systems based on diverse agricultural practices, rehydration, polyculture, conservation livestock practices and the production of ecological goods and services, will be sustainable in the tight definition presented.
As water management ,including water harvesting, irrigation, soil infiltration and reforestation, is of utmost importance to food production, it can be concluded that soil rehydration in its widest interpretation will support diversity farming for food security within a defined area and specified (and supporting) population base.
Designed land use systems where rehydration supports diverse agriculture towards food security have been implemented and are proven to be successful. The World Bank’s TerrAfrica project is an example of land reclamation and rehydration of lands on some 25million hectares of Ethiopian land to benefit 30 million people. Similar projects have been shown in WOCAT documentation co-produced with TerrAfrica. Through a combination of excluding free ranging livestock from the landscape, terrace building on steep slopes, water capture and infiltration in soil on medium slopes for growing cash crops, aquaculture in ponds and reforestation, and slowing water run-off in highlands with gabions etc., a sustainable system has been set up there to include agricultural diversity and resulting food security. This project has been fully supported by all levels of government, with local planning and community involvement.
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