Water use in agriculture can be directly related to plant productivity or indirectly to animal productivity through the consumption of grain or forage. The major challenge to agriculture is the production of 50% more food by 2030 and the doubling of the world’s population by 2050 (OECD, 2010). The assumption is that this need will have to be met with less water because the increasing population will increase their demands for water for both domestic and industrial uses. The agricultural sector has evaluated water from a singular view and focused on the water requirements of crops and minimized the discussion about the impacts of water moving past edges of the field or below the rooting zone. Hydrologists have examined the overall water balance of watersheds and basins and FAO (2012) reported the global total of all water use was 70% in agriculture, 11% in municipal uses, and 19% in industrial systems. The large use of water by agriculture for production raises the question as to whether agriculture could become more efficient in its use of water and reduce the environmental impact (OECD, 2012, Young, 2010).
Environmental impacts of water use are multifaceted and encompass a range of factors. We can consider the water balance to be a function of the distribution of precipitation into base flow of streams or rivers, groundwater recharge, surface runoff, and evapotranspiration, which can be further divided into evaporation from the surface and transpiration through the plant. In agriculture, the significant use of irrigation to meet water demands not directly supplied by precipitation has to be considered as a component of the water balance. However, from an environmental impact all of the components of the water balance have a significant role and are part of the overall water balance in agriculture and cannot be considered in isolation from one another. One challenge we are presented with is to consider the environmental impacts of the agricultural water balance from the viewpoint of developing and implementing practices which will enhance environmental quality while maintaining or enhancing productivity. These are not new questions; however, given the increase in the demand for fresh water, understanding the potential to improve environmental quality is key component to the sustainable use of our natural resources.
Water transpired through plants or evaporated from plant surfaces, the soil, or water surfaces enters the atmosphere and has an environmental impact; however, the implication of this portion of the water balance is often overlooked. It has long been recognized that the process of evaporation is a cooling process. As water is evaporated there is a cooling effect because the energy consumed in the process of evaporation reduces temperature of the air surrounding the object and the temperature of the surface on which evaporation is occurring. Cooling of leaves through evaporation has often been considered as a microclimate effect in which the impact of the latent heat process is an extremely local process and cooling only occurs in a small volume of air. However, Ban-Weiss et al. (2011) suggested that the large scale effect of evaporation may be more noticeable than once thought. In their analysis they found for every 1 W m–2 of latent heat coupled with a sink of 1 W m–2 of sensible heat there would be a mean decrease in global temperature of about 0.5°C (Ban-Weiss et al., 2011). An environmental impact of evapotranspiration is to maintain the air temperature at cooler levels than without evaporation from agricultural surfaces. We often ignore this environmental impact but it may be one of the most critical factors in evaluating the effectiveness of considering changes in agricultural water management. Reduction in the amount of water used in agriculture may have an impact on the overall temperature of the earth because of the reduction in the potential cooling.
Surface runoff from agricultural systems is often considered to be an indicator of poor land management because the soil is not able to absorb all of the precipitation during a particular storm; however, there are frequently instances in which the rate of precipitation exceeds the ability of the soil to absorb water. There are obvious environmental implications of runoff because of the movement of sediment and transport of nutrients and pesticides from fields into nearby water bodies. Water runoff during precipitation events begins to occur when the precipitation rate exceeds the infiltration rate into the soil. During a precipitation event, as the soil at the surface becomes saturated, the integrity of the soil aggregates begins to diminish and the infiltration rate decreases. This response can occur when the storm intensity exceeds infiltration rates or the prolonged impact of raindrops on an unprotected soil surface begins to cause displacement of the soil particles. The ability of an individual storm or series of precipitation events to cause soil erosion, or rainfall erosivity, is correlated with the effect of storm energy interacting with the maximum prolonged precipitation intensity (Nearing et al., 2005). Soil management practices which protect the soil surface from the force of the raindrops will have a positive impact on reducing the amount of raindrop energy directly on the soil surface.
Management of the soil surface to maintain crop or residue cover to protect the soil surface from raindrop energy can reduce surface runoff. In many instances, surface runoff occurs at times of the year in which there are small rates of evaporation from the soil surface or crop water use causing the soil profile to have little capacity for storing additional water. In those instances it is imperative to ensure that surface water moving from the field is managed to either reduce the velocity in channels and flows into areas in which the erosive force of the water can be minimized. Runoff from agricultural systems is often considered a negative environmental impact; however, movement of runoff is often the water source for lakes, streams, and rivers to maintain their functionality. The environmental impacts of surface runoff extend beyond the displacement of soil and the resultant sediment load and include any material which may be attached to the soil. Movement of P and pesticides from agricultural fields has often been considered to be the primary components of environmental concern from agricultural fields; however, this has to be broadened to consider pathogens and antibiotics from unprocessed organic fertilizer applications.
Groundwater Recharge and Leaching
Movement of water through the soil profile occurs when the amount of water exceeds the storage capacity of the soil and water moves downward under gravitational force. The amount of water moving through the soil profile will depend on a number of factors, for example, soil water holding capacity, evapotranspiration rate, precipitation pattern, and substrate materials. Soils differ in their water holding capacity and a sandy soil will have more leaching through the profile than will a clay soil; however, the largest factor is the temporal dynamic combining the precipitation pattern with the crop water use pattern. In irrigated crops, the interaction of irrigation amount and timing with the crop water use patterns must also be considered. Leaching through the soil profile occurs when the precipitation amount exceeds the crop water use rate leading to an excess in the soil profile. When leaching occurs, soluble materials also move with the water through the soil profile, most notable of the soluble materials is nitrate N. Nitrate-N concentrations in subsurface drainage lines are indicators of leaching through the soil profile and the patterns during the year have been related to drainage or leaching through the soil profile. These patterns are often most pronounced in the upper Midwest where there is extensive use of subsurface drainage and the soil water recharge is complete in the early spring before the crops begin to remove water from the soil profile. Leaching of nitrate N through the soil profile has been one of the most extensively studied compounds affected by the soil water balance and agricultural management. An analysis of the Raccoon River Basin in central Iowa by Hatfield et al. (2009) showed the increases in nitrate-N concentration were related more to changes in crop water use patterns caused by shifts in cropping systems more than changes in fertilizer use patterns. The shifts in the cropping patterns were related to the changes in the amount of baseflow in the Raccoon River suggesting a strong linkage between cropping patterns and groundwater recharge (Schilling and Zhang, 2004). These patterns are of concern because of the potential implications of nitrate-N concentrations in drinking water supplies.
The environmental impacts of leaching of compounds through the soil profile into the drainage water or ground water will depend on the material being transported and the concentration. There is increased concern about the movement of chemicals, pathogens, and antibiotics through the soil profile. Understanding the movement patterns will require that we couple the processes linked with the occurrence of these compounds in the soil profile with the patterns of crop water use, precipitation patterns, and soil water holding capacity.
The impact of water management on the environment is given through the use of irrigation and the infrastructure associated with water management practices. There are approximately 22 million hectares of irrigated land in the United States using a combination of ground and surface sources and the environmental impact of irrigation could be divided into two aspects; the direct impact resulting from the process of irrigation; and the indirect resulting from the infrastructure associated with the change in the landscape, for example, water reservoirs, canals, drainage ditches, field leveling, etc.
The direct environmental impacts from irrigation on the regional climate may actually be larger than in rainfed environments because the effect of increased evapotranspiration in an environment which would be much warmer without the cooling effect via evaporation. The impact estimated by Ban-Weiss et al. (2011) may be conservative for irrigated areas especially those in arid climates. Another environmental impact from irrigation is the potential for increased surface runoff in areas with furrow or border irrigation. The addition of irrigation water could increase deep percolation of nutrients from the root zone, increase soil salinity or sodicity, and in some areas lead to an increase in the water table height. Young and Wallender (2002) demonstrated at the water district level in California that irrigation changed the groundwater recharge and these amounts varied with position on the landscape and slope position would be an important factor in evaluating the effect of irrigation on hydrologic processes. Zhang et al. (2012) showed that development of irrigation in river basins would reduce surface runoff from a river basin suggesting that irrigation would alter the water balance in a landscape.
Environmental impacts from the infrastructure associated with irrigation may have even a larger effect because of the reshaping of the landscape. The construction of reservoirs and canals for the delivery of surface water to fields alters the natural flow of water in the landscape and has created artificial wetlands in positions on the landscape which were not present before irrigation (Sueltenfuss et al., 2013). The impact of irrigation on landscape processes does create changes in potential environment impacts and in semiarid regions would potentially change the distribution of wetlands on the landscape. One example of this effect is provided by Lemly et al. (2000) in which they showed that the increased demand for water for irrigation reduced the wetland area and subsequently affected the habitat area for wetland birds. The alteration of the water balance through the use of irrigation does have environmental impacts beyond the edge of the field and these effects must be considered in evaluating the role of agricultural water management on environmental quality.
The environmental impacts of crop water use cover all of the components of the overall water balance. There are both positive and negative impacts of soil water balance because of the timing and magnitude of water movement into the atmosphere, surface water, or ground water. The amounts transported into surface water and ground water depends on the overall water balance at any point in time during the year. There is a balance among the seasonal precipitation patterns, crop water use patterns, and the soil water holding capacity that affects this balance. To reduce the environmental impacts from this imbalance requires that soil and agronomic management practices be implemented to reduce potential impacts. These include:
Maintenance of crop residue cover on soil, especially during heavy precipitation events to protect the soil surface from the raindrop impact and maintain infiltration rates into the soil.
Implement soil management practices, for example, grassed waterways, vegetative buffers, etc., which reduce the erosive forces from moving water if it leaves the edge of the field.
Utilize cover crops to protect the soil surface but to also extract water and nutrients from the soil to reduce both leaching and surface runoff.
Implement more diverse crop rotations to alter the seasonal water use patterns relative to the precipitation patterns.
Implement best management practices for irrigation to enhance water use efficiency and protect water quality.
Optimize fertilizer and manure application timing, rate, form, and placement.
Implement landscape management practices, for example, buffer strips, biofilters, controlled drainage systems, etc., to provide a potential method of capturing sediment, nutrients, pathogens, pesticides before being transported into streams, rivers, or lakes.