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ABSTRACT

Several areas in Oregon, USA, are susceptible to nonpoint source groundwater contamination from crop production under irrigation. Efficient use, and economical reuse, of water in irrigation is the key to protecting the quality and quantity of shallow groundwater. Soil salinity is not a major problem in these regions.

Agriculture has been concerned almost exclusively with effectively using inputs of energy to achieve maximum production. Now the focus has shifted to management of resources for preservation of quality of water and land. Various incentives and regulations are reinforcing this change in Oregon. The management strategies for protection of water quality through efficient irrigation include timing and rate of application of chemicals, more efficient irrigation methods and application techniques, and reuse of irrigation water. Groundwater containing nitrate is being reused for irrigation, and has value as both fertilizer and water.

Conservation is seen as the key to efficient water use. Conservation makes water available for other beneficial uses. While simple in concept, application of conservation measures impact on economics, social patterns, and institutional arrangements for allocation of water, as well as on the nature of our needs for water. Because water is necessary for life, substitutions for a scarce resource are limited, price cannot be used to ration water because of social inequities, and national policy could require use of water for lower value uses such as agriculture.

INTRODUCTION

Irrigation is used to grow crops in areas where the natural rainfall would be insufficient to meet crop water needs, and is also used to decrease the variability in crop growth and yield from year to year. Irrigation systems to apply the water can be fairly expensive, with annual costs ranging from several tens to several hundred U.S. dollars per hectare. Therefore, it is necessary to maximize other inputs for crop production, such as use of fertilizers and pesticides. These inputs of off-farm energy have led to large increases in agricultural production during the past 50 years, but the inputs are expensive. The water itself may be relatively inexpensive. Where this is true, there is little economic incentive to use water efficiently. However, where water costs are a significant proportion of the total crop production costs, water use efficiency becomes a concern in crop management.

Irrigation is generally practiced on medium-grained and coarse-grained soils, sands or sandy loams, which generally have a high permeability. Under irrigation for maximum crop production, the top layer of soil is usually kept moist, and therefore the crops develop shallow root depths of the order of 30 to 60 centimeters. High permeability and shallow root zone depth make leaching below the root zone a common occurrence.

In the drier areas of Oregon where irrigation is practiced, annual rainfall is in the order of 25 cm, with most of it falling during the winter. As in other climatic regions with low rainfall, the variability from year to year is high: annual rainfall varies from 15 to 45 cm.

The connection between irrigation and quality of groundwater is most direct through the process of leaching of soluble chemicals below the root zone. Nitrate carried below the root zone moves downward in pulses during years of extra rainfall or from over-application of irrigation water beyond that stored in the root zone. Soluble pesticides can also be leached. These pulses may move soluble materials at average rates of 10 to 100 centimeters per year.

Excess irrigation water is used in arid areas to leach salt from the root zone. This leaching fraction and any unplanned additions of extra water from over-irrigation or high rainfall contribute to shallow groundwater. Rising water tables are a severe management problem in irrigated agriculture in arid areas. Gradual salinization is often due to rising water tables where proper drainage has not been provided.

Ideally, the amount of irrigation water applied should exactly equal the water used in evaporation and transpiration through the crop. Various practical aspects of irrigated crop production make it difficult to achieve this goal. It is achieved most closely in drip irrigation, less so with sprinkler irrigation and furrow irrigation, and least of all in flood irrigation. Efficiencies in water use can be achieved by going to sprinkler or drip irrigation, but the energy costs of doing so also increase in this direction. Limitations due to energy costs will decrease this trend in the future. At the same time, major improvements in efficiency are occurring in irrigation systems that apply water in furrows.

High production of crops has been achieved in the past 50 years through the use of energy inputs in agriculture. The interest now is in increasing efficiency. For reasons of economics and environmental quality, there is now a need to achieve greater efficiency in crop production with lower inputs. The key to lower economic and environmental costs is conservation.

IRRIGATION EFFICIENCY

The amounts of water used for irrigation are large compared with the amounts used domestically. Small gains in efficiency in irrigation would produce significant increases in water available for other uses. Efficiency is important where competition for water makes it desirable to increase water supplies. Efficiency also decreases the energy costs of pumping and providing water for irrigation. Competition between irrigation and domestic uses is usually decided in favor of the latter, because the economic willingness to pay is higher for domestic uses. The allocation of water in southern California, U.S.A., in recent years is an example. However, irrigation and in-stream water uses such as fish habitat, recreation or aesthetics (e.g. wild and scenic rivers) are more equal, especially recently as public uses of water are being more vigorously defended. Efficiency of use (conservation) is being seen as a source of extra water for these desirable public uses.

Irrigation efficiency has several different components (Table 1). Losses in the distribution systems are due to leakage and evaporation; losses during application to the field are due to wind, evaporation and runoff, and losses from the soil are due to excess water, applied beyond what the crop uses.

Efficiencies can be increased in different ways. Losses in distribution systems can be avoided by using pipes or by lining irrigation canals. Application efficiency varies with the irrigation system, and can be increased, for example, by changing from furrow to sprinkler or drip irrigation systems. Irrigation scheduling can be used to determine exact amounts of water needed by the crop.

A less obvious factor in irrigation efficiency is the effect of soil variation (English, et. al., 1986). Table 2 shows a hypothetical example where different parts of a field hold different amounts of water in the root zone. Different management strategies can be used to determine amount of water added, which results in different efficiencies. The average root zone capacity is 11 cm of water. If this amount is added, there will be leaching in half the field, and the other half will not reach capacity. While the nominal water use efficiency is 100%, the actual efficiency is 91%. Efficiency is defined as the proportion of the water applied that is stored in the root zone. If 14 cm of water is added, the amount needed to fill the entire field to root zone capacity, leaching occurs in three quarters of the field.

A further variation under furrow irrigation is the different amount of water applied near the top of the furrow, which is wet for a longer time, vs. that applied near the bottom of the furrow. This is true even for a uniform soil, and increases as length of furrow increases. “Surge” irrigation, with a larger initial flow, can increase the efficiency.

The cost of water influences the irrigation efficiency achieved in practice. Table 3 shows a comparison for two areas in Oregon with different costs of water (Miglioretto and Warkentin, 1990). The area with a high cost of water uses only as much water as the calculated consumptive use, the area with inexpensive water uses 50% more.

* For a crop of white potatoes.

EFFICIENCY AND GROUNDWATER QUALITY

Simple models indicate that soluble chemicals such as nitrate will be leached below the root zone in proportion to the volume of water draining out of the soil (Jury and Nielson, 1989). If nitrogen is available for optimum growth of the crop, nitrate loss will be linearly related to leaching volume. These simple models are based on the assumption that solubility is not limiting, and on equilibrium. They turn out to be useful approximations for most soils. This linear relationship holds regardless of soil variability and irrigation uniformity. As long as nitrogen is available, any excess water will move nitrate.

Timing of nitrogen applications will affect loss by leaching because it affects concentration of nitrate in the soil solution at any particular time. The largest loss will be where all of the nitrogen is applied at planting time. Water quality can be protected with a management system that limits the nitrate in the soil to the amount needed at any time by the crop.

A 1 cm loss of water to leaching below the root zone could result in a loss in the order of 5 kg ha-l of nitrogen, if the soil solution concentration is in the order of 50 mg l-l. Over several irrigations, and with several centimeters of leaching, the losses of nitrogen can approach 100 kg/ha. The cost of the nitrogen lost could be US$45; the cost of water lost would be US$10 - $50.

Gains in both groundwater quality and in irrigation efficiency can be obtained if fields are managed differently depending upon the soil variation. If the variation occurs over a distance of approximately 50 meters, different amounts of water can be applied to different parts of a field. If this variation occurs over 5 meters, it is usually not practical to treat different parts of the field differently. The nature of soil variability then becomes an important factor in management for efficiency and groundwater quality. Such systems require greater use of knowledge as well as more sophisticated technology.

WATER REUSE

Some of the irrigation water that is lost during conveyance and application to the land may be reused. Reuse is fairly common. The nature, and the amount of reuse, depends upon local conditions. Runoff from the ends of irrigation furrows may be collected and used as a source of irrigation water for fields at lower elevations. Pump-back systems that collect water running off the ends of fields are also fairly common. Under some conditions, excess water may return by natural flow to surface water bodies or to groundwater, and be used again. In each of these cases the actual efficiency of use would be increased when this is taken into account. However, water quality is often decreased, which may limit the reuse of water. Sediments and soluble chemicals in the water may make it unsuitable for reuse. The interaction between efficiency and water quality is complex, but in general, higher efficiencies will result in higher quality.

A particular case of reuse is using groundwater with high nitrate as irrigation water. Groundwater containing nitrate at concentrations in the range from 10 to 50 milligrams per liter is pumped from depths of 10 to 30 meters and used as irrigation water in Oregon. At nitrate concentrations below 25 mg l-l, extra nitrogen has to be added to meet the crop requirements. Above that concentration, excess nitrogen is supplied if irrigation is based on water requirements of the crop. Water with high nitrate concentrations is therefore mixed with low-nitrate water to achieve the correct amount of nitrogen. The value of the nitrogen offsets part of the pumping costs (Table 4).

* Calculated for a maize crop with a requirement of 200 kg ha-l of nitrogen and 70 cm of irrigation water.

GROUNDWATER PROTECTION STRATEGIES

The Groundwater Quality Protection Act in Oregon is designed to protect, conserve and restore groundwater resources. Groundwater quality surveys provide the information for a database to identify areas where groundwater quality is at risk. If the groundwater has nitrate concentrations in excess of 70% of the allowable level for nitrate, or concentrations of other chemicals above 50% of the appropriate allowable level, the area is declared a Groundwater Management Area. Two such areas currently exist in Oregon, with nonpoint source contamination of groundwater. A local Groundwater Management Committee composed of interested citizens and water users in the area is appointed. They prepare an action plan which details the voluntary implementation of “best management practices” and use of individual farm management plans. The plan is based on information available from research studies. The plan includes monitoring for effectiveness, and it details the research, information transfer, education and demonstration projects that will be required for implementation.

These “best management practices” include strategies for irrigation efficiency. Since any excess water will carry soluble chemicals below the root zone, efficiency in water use is the key to maintaining and improving groundwater quality. These strategies become the components of individual farm plans which detail the irrigation scheduling, leveling, application systems, etc. that will be used to increase irrigation efficiency.

BENEFICIAL USES

The purpose of water management is to provide water for beneficial uses. The amounts needed to meet beneficial uses vary with the region, so it is difficult to use a uniform definition over large areas. There can be general agreement that any use beyond the amounts required is waste, but general regulations defining waste would be difficult. This results in disagreement over how much water is wasted and by how much water supplies would be increased by conservation.

In Oregon, the competition for water has led to increased interest in conservation. The water conserved would be put to other beneficial uses. Rainfall is unevenly distributed, with winter rainfall and dry summer periods, so water storage is an issue. Water use is regulated through an appropriation doctrine which allows removal of water in amounts sufficient for beneficial uses. Priority for water use is based on date of filing for a water right. During periods of low surface water flow, holders of rights with later dates will be the first to be restricted when water flow cannot meet all of the appropriated rights. Water quality was not an original concern in the appropriation doctrine, but has become a major concern now. The question is raised as to whether a beneficial use can allow a decrease in water quality.

REFERENCES

ENGLISH, M.; TAYLOR, A. and JOHN, P. (1986), “Evaluating Sprinkler System Performance.” New Zealand Agricultural Science 20:1.

JURY, W. A. and NIELSON, D. R. (1989), “Nitrate Transport and Leaching Mechanisms” in Follett, R.F. (ed.) Nitrogen Management and Ground Water Protection, Chap 5. Elsevier, Amsterdam.

MIGLIORETTO, T. and WARKENTIN, B. P. (1990). “Northern Malheur County Water Management Area.” Unpublished Report, Department of Soil Science, Oregon State University, pp 106.