In this module, you’ll look at what happens when water is out of balance naturally or is not in line with the needs of the agricultural production in a given climate. You’ll also look at some strategies from agroecology to reduce needs or increase efficiency of water use in an agroecosystem or farm/garden.
In natural ecosystems, vegetation adapts to the soil moisture regime of the climate and the soil type. In agroecosystems, sometimes we introduce plants that have different water needs than the ecosystem has the ability to provide for naturally. Irrigation (artificial application of water to the land or soil) provides adequate soil moisture for the crops introduced that have water needs beyond the average rainfall of the season. Irrigation is also used to assist in different methods of growing of agricultural crops, maintenance of human-managed landscapes, re-vegetation of disturbed soils in dry areas and during periods of inadequate rainfall.
Consequences of Irrigation
While there is evidence that irrigation has been practiced by humans for thousands of years, irrigation represents a major change in ecosystem function and generates its own ecological problems. Water supply systems are also both costly in terms of money and energy. The ecological and economic costs of irrigation must be considered when considering the long-term sustainability of a farming system that uses irrigation.
Excerpt 1: “Depletion of Water Resources”
by Paul Nabhan | The Desert Smells Like Rain: A Naturalist in Papago Indian Country
Pima schoolgirls, ca. 1900
While the Papago perceive the rain is dying, Anglo dwellers in the Sonoran Desert are now realizing that another water resource has been driven into a death march: groundwater reserves accumulated during the Ice Age are rapidly being depleted. As Chuck Bowden says in Killing the Hidden Waters, it has taken less than a century of groundwater pumping for modern Southwestern agriculture to deplete fossil water reserves that cannot now be recharged, and this has been done at the expense of pressure fossil fuel as well. Seventy miles north of the reservation, water levels are dropping as much as twenty feet per year, due to a pumping rate nearly one hundred times that of natural recharge. In many places throughout southern Arizona, the cost of pumping is greater than the value of certain water-consumptive crops. Mexican arid lands expert Enrique Campos-Lopez predicted that by 1985, rainfall-based agriculture will again be more energetically attractive than mechanized, groundwater-irrigated agriculture on both sides of the border. Desert runoff farming is suddenly the “new idea” for hydrologists and crop scientists who live within an hour of Papago floodwater fields, but have never ventured out of their offices to see them.
Concomitant with the resurgence of interest in rainfall and floodwater harvesting is a new appreciation of the value of desert-adapted crops. Yet many of the traditional drought-hardy crop varieties fell out of use and became extinct when commercial agriculture based on pumping and hybrid crops was initiated earlier in this century. Papago, leaving their floodwater fields to work for wages in irrigated fields, lost many of their bean and corn varieties as the life of their remaining seeds expired while they were away. At the same time, their water control structures deteriorated, and the washes which fed them were disrupted by new roads and livestock ponds.
As Daniel Janzen points out, “What escapes the eye…is a much more insidious kind of extinction: the extinction of ecological interactions.” Not just crops were lost- whole fields systems atrophied. Roughly 10,000 acres of crops were grown via Papago runoff farming in 1913; by 1960, there were only 1,000 acres of floodwater fields on the Papago Indian Reservation. Today, Papago sporadically farm less than 100 acres using floodwaters.
While the remaining acreage is minuscule, it is all that is left of an ecologically sensitive subsistence strategy that has endured in deserts for centuries. Here, not only a rich heritage of crops remains, but also co-evolved microorganisms and weeds, as well as pests and beneficial insects. Amaranths, for instance, are hosts for insects that control corn-loving pests. Papago fields harbor nitrogen-fixing bacteria which naturally associate with tepary bean roots. A species of solitary bee has been found visiting annual devil’s claw in Papago fields, but despite a thorough search has not been found on wild annual devil’s claw elsewhere. Moreover, there is a mutually beneficial relationships between these plants and their Papago stewards; the Papago have evolved field management skills that have allowed them to sustain food production for centuries without destroying the desert soils. The plants have evolved the ability to grow quickly, root deeply, disperse heat loads, and provide nutritious seeds for those who harvest them. These durable functional relationships between humans and other lifeforms are the products of a slow evolution and cannot be remade in a day. No amount of academic research on water harvesting and drought-hardy crops can replace a time-tried plant/man symbiosis such as that in which the Papago have participated.
Water harvesting, storage and delivery can have major impacts on surface and subterranean water flow. Aquifers can be over-drafted, and the ecology of river, riparian and wetland ecosystems can be damaged. Since maintaining healthy waterways and water supplies is as important as maintaining profitable crop production, the impacts of water-supply systems on local and regional hydrology must be taken into account.
These pictures show the consequences of salinization due to excessive irrigation.
Nearly all irrigation waters contain salts that can damage soils and crops if allowed to accumulate. Since irrigation is primarily used in areas with a high potential for evaporation, the salts deposit on the soil surface and build up over time. If uncontrolled, this buildup of salts, known as salinization, can reach levels unfavorable for crop production, especially when the salts contain toxic trace elements like boron and selenium.
Salt buildup and resulting salinization is inevitable in most irrigated systems, long-term sustainability isn’t possible without adequate natural or artificial drainage that removes accumulated salts from the upper soil layers. Rainfall is the primary leaching agents, but without sufficient rainfall, it is necessary to construct systems of drains, ditches and canals as described above. A practice often used in conventional cropping systems is the application of excess irrigation water to dissolve the salts. However, this water then carries appreciable salt loads since the water leaving the agroecosystem has a higher salt concentration than the water applied. This practice can contribute to salinization of the areas receiving the water flowing out, the groundwater or surface waters systems, and it can damage natural ecosystems and destroy other agriculture fields.
Other ecological changes due to irrigation
When irrigation is introduced into a farming region during a normally dry part of the year, it can have profound effects on natural ecological cycles and the life cycles of both beneficial and pest organisms. Under natural conditions, seasonal drought may have been an important means of reducing the buildup of pests and diseases, acting much as a frost or flooding does in regions to reduce the life cycles of these organisms. Loss of this natural control mechanism can have serious consequences in terms of outbreaks of pests and increased resistance to artificial pesticides.
Managing Water Deficit and Temperature in Agroecosystem
Adapted from Gliessman | Agroecology 2nd ed., 2007
Crop Choice and Agroecosystem Design
The choice of plant species and the timing of cropping is extremely important for soil moisture management. If soil moisture is limited, and/or there is high evaporation and/or limited water for irrigation, a farmer can choose a crop or variety with less-intensive water needs, or shift the growing of more water intensive crops to a cooler time of the year when moisture loss potential is less.
Tomato Fields in Summer in Dalat, Vietnam>
Observe the next two environments, which do you think suffers from more evaporation?
It is probable that a) suffers from more evaporation. Below you will find out why, and you will also see some tips to design your agroecosystem to avoid unnecessary loss of water.
Intercropping involves planting crops between rows in perennial or annual production. Intercropping can increase yields in a given acre, for instance herb, vegetable or animal production between rows in an orchard. In vegetable production, different crops can be planted together to reduce evaporation. Since dark soil absorbs more heat, by keeping the surface covered with plants reduces the heat absorption, reducing the soil temperature and soil moisture evaporation.
Evaporation from the soil surface results in the loss of more than half the moisture gained from precipitation. This occurs in dry-land regions, irrigated and rain-fed humid regions. Plant growth suffers as a result of the loss of moisture through surface evaporation. Practices that cover the soil aid in the reduction of water loss by evaporation. Fallow cropping is when a farmer plants a cover crop to cover the soil and reduce evaporative losses when the field is not being used to produce an economic crop (although some cover crops can be economic crops.)
Both plant and animal materials can be used to cover the soil surface in order to reduce evaporation. As an added benefit, this mulching reduces weed growth. Materials for mulching can be sawdust, leaves, straw, composted agricultural wastes, manure and crop residues. Mulches provide a barrier to moisture loss, and have special value in high-value crops such as strawberries, blackberries, and some other fruit crops. Mulches work best when the cropping system requires only infrequent cultivation or relies mostly on hand weeding.
Mulching provides a viable option for soil water management, while at the same time it has many other beneficial effects. It protects the soil from erosion, returns organic matter and nutrients to the soil, alters the surface reflectivity (albedo), creates a boundary layer against gaseous diffusion, and allows better infiltration of incoming rainfall. All of these factors interact with one another.
Green manures: can be cut and then plowed into the soil or simply left in the ground for an extended period. When green matter is left over after a crop is harvest, it is incorporated back into the soil, recycling the nutrients back into the soil food web. However, green manures can require increased tillage, so it may break up soil structure and habitat for soil biota.
When residues from the cropping season are left on the surface of the soil it creates what’s called “crop residue, a protective barrier that lowers the evaporation potential while keeping the soil temperature lower, further reducing evaporation.
Reduced Tillage and No-Tillage
Reduced tillage and no-tillage often use crop residues as mulch. A major goal of most reduced tillage systems is to develop greater soil cover to reduce evaporative losses from the surface. In no-till systems, seeds are sown directly into the residues of the previous crop. Crop residues are left on the soil surface, forming a mulch that modifies the temperature of the soil and prevents moisture loss.
Natural soil mulch made from a cultivated dry soil layer on the surface of the soil can conserve moisture in region with distinct alternation between wet and dry seasons. This dry layer breaks the capillary flow of water to the surface, and the process of its creation eliminates weeds that might tap moisture below the dry layer and increase transpirational losses. However, these positive outcomes must be weighed against possible negative impacts such as increased labor costs, and a greater threat for soil erosion.
There is a range of specially manufactured papers and plastics for use as mulches. These materials are spread out and firmly secured to the soil surface, and are spread directly over prepped beds. Slits or holes in the material are made for the to plant the crops into. These mulches reduce evaporation and weed growth, often resulting in increased yields. Plastic mulches in particular can raise soil temperatures by several degrees. This is an important benefit for crops planted during the colder time of the year.
The problem with artificial mulches is that they are not produced on the farm, and often they are reliant on fossil fuels to be produced. They come at a high cost economically and environmentally. When they have been used for a few seasons, they begin to deteriorate and must be disposed of. This is an example of some of the hard trade-offs many sustainable farmers have to consider.
Modifying Temperature Micro-climates
Through appropriate design and management, the microclimate of an agroecosystem can be modified to create or maintain micro-climatic conditions that favor the sustainability of the cropping system. Each modification should be evaluated for its contribution to short-term yield and market return as for its contribution to the longer-term sustainability of the system. Micro-climates include many factors, but the modification of micro-climates focuses mainly on temperature. These practices can also impact other factors of micro-climate, such as humidity and light.
Canopy Vegetation/Agroforestry Systems
Trees or other tall plants that create a canopy over the other plants in a system can greatly modify the temperature conditions under the canopy. Shade from the canopy reduces solar gain at the surface of the soil, as well as helping the soil retain moisture. Agroforestry systems in the tropics are a good example of this kind of practice.
There are other ways of creating a canopy for a cropping system. Floating row covers of nylon are used to protect crops from bugs and keep them warmer during cold months. It insulates the soil surface, and provides a localized green house effect for re-radiated heat given off from the soil surface. Floating row covers can also prevent evaporation, and trap moisture.
There has been considerable research and practical experimentation in the use of “hoop houses” or high-tunnels for vegetable production. Wire or plastic hoops are placed over planted beds in the field, and then covered with plastic or cloth. The localized greenhouse effect of these structures traps and holds additional heat during the day, and the covering reduces heat losses at night. Hoop houses can allow for the earlier planting of warm-weather crops such as tomatoes or peppers, or the extension of the cropping season into the fall or early winter where light frost becomes possible. However, they can be expensive, which limits their use.
Soil Surface Cover
Intercropping and mulches cover the soil surface, which both reduces evaporation and changes the soil temperature. The temperature change depends on the thickness and color of the cover. Mulches can be made out of any living plant material, so long as the plant doesn’t have any seeds that could lead to a weed problem later, or allelopathic effects from the plant . Plant derived mulches eventually get incorporated into the soil, benefiting soil organic matter content.
Temperature and Sustainability
Designing and managing agroecosystems that are sustainable with regard to temperature factors involves two interrelated challenges. The first challenge is to deal with the temperature factor in ways that are not overly reliant on external inputs or the use of fossil fuels, do not harm natural systems or diminish genetic diversity, and do not exacerbate inequality in the social sphere. This aspect of sustainability puts limits on the use of structures like shade houses, materials like plastic sheeting, and other technologies and shifts the focus to efforts that provide micro-climate modification as a feature of agroecosystems’ basic design. In this latter category are agroforestry systems that create a diversity of micro-climates in their interiors and work to moderate temperature extremes.
The second challenge is to create a production systems that can withstand the rising temperatures, temperature extremes and seasonal temperature anomalies that will increasingly confront farmers over much of the world in the coming years. The keywords in this effort are adaptation and resilience. Adaptation involves an ability to change management strategies, crop types, seasonal timing, and agroecosystem design in response to changes and anticipated changes in the temperature regime. Resilience comes from designing systems that are inherently less vulnerable to temperature extremes and variability, able to recover from damage, and diverse enough to yield food no matter what kind of weather they are subject to.
Ultimately, these two challenges come together. Agroecosystems that can survive climate change are also the ones that do the least harm to the ecological foundations of agriculture: they leverage diversity and natural processes, and they are designed and managed based on knowledge of the environmental context that includes very centrally the factor of temperature.
Gliessman, Stephen R. Agroecology: The Ecology of Sustainable Food Systems. 2nd ed. Boca Raton: CRC Press, 2007.