Journal of Irrigation and Drainage Engineering

The Task Committee on Water Quality Problems Resulting from Increasing Irrigation Efficiency coordinated preparation of a series of papers that addressed the perceived effects on water quality from increasing irrigation efficiencies. This paper summarizes and integrates results presented in the requested papers, which deal primarily with salinity and nitrogen relationships, and also analyzes other ramifications of increasing irrigation efficiency. Increasing on‐farm efficiency usually connotes providing water to meet crop needs with minimal deep percolation and surface runoff. Topics examined include leaching requirements, chemical reactions on the root zone, potential hazards and benefits of reduced leaching, nitrate leaching, basin‐wide implications of increasing efficiencies, and effects on groundwater quality. Effects on erosion and the movement of pesticides are also considered. Physical changes to improve irrigation efficiency cannot be considered independently of the associated economic, social, legal, and institutional restraints. It is concluded that from a water quality viewpoint there is no reason to avoid increasing irrigation efficiency, but any such project should be analyzed thoroughly in advance to determine whether local circumstances warrant the improvements.

The amount of drainage required to prevent loss in crop productivity from excess soil salinity has been termed as the leaching requirement $\left(L_r\right)\text{.}$ Steady‐state models for predicting $L_r$ are reviewed and compared with experimentally measured values. Values of $L_r$ predicted from the exponential model, which computes the average soil salinity in the root zone based upon an exponential pattern of crop water uptake, correlated best with measured values. Acceptable estimates are also obtained with a simplified model that approximates an average soil salinity based on values for the top and bottom of the root zone. The highest correlation coefficient was 0.67, indicating that none of the models is completely satisfactory.

The quality of an irrigation water, in both its total salinity and individual constituents, influences the chemical character of the soil solution and irrigation return flows including drainage waters. Dissolution, precipitation, adsorption and exchange reactions modify the salt load and composition of return flows. The effects of these reactions can be manipulated by varying the leaching fraction. A change from significant increase in total salts to substantial precipitation of calcite (soil lime) and gypsum can occur as the leaching fraction is reduced. Such a reduction in ieaching fraction and the concomitant reduction in applied water will also reduce the rate at which native salts in the return flow pathway are displaced into receiving waters. Methodology exists, as illustrated by several examples, to evaluate potential consequences of reduced leaching and chemical reactions on soil solution composition and on the salt loads of irrigation return flows.

Increasing on‐farm irrigation efficiency reduces deep percolation and generally increases concentrations of soluble salts in the root zone. Since more salt precipitation occurs within the porous soil matrix, the mass of soluble salts is reduced. In addition, exchangeable sodium builds up on the cation exchange complex and plant available water decreases. Conventional irrigation scheduling techniques become less adequate and the need for salinity monitoring increases. Generally, however, the potential benefits of increasing irrigation efficiency outweigh the potential problems caused by the aforementioned.

The literature was reviewed to determine the effect increasing irrigation efficiencies have had on ${\mathrm{NO}}_3-N$ leaching. The mass of ${\mathrm{NO}}_3-N$ leached is directly related to the drainage volume. By increasing irrigation efficiencies, both the drainage volume and amount of ${\mathrm{NO}}_3-N$ leaching are reduced. Both water management and nitrogen management are important in controlling ${\mathrm{NO}}_3-N$ leaching. By using improved water management practices that control the amount of water applied and using the proper time to apply water, the irrigation efficiency is increased and ${\mathrm{NO}}_3-N$ leaching is reduced. Applying only enough N fertilizer to meet crop requirements for a realistic yield goal, time of application and use of slow release fertilizers are N management practices that will reduce ${\mathrm{NO}}_3-N$ leaching. However, it is impossible to reduce ${\mathrm{NO}}_3-N$ leaching to zero in the humid region on coarse‐textured soils and still maintain adequate crop yields. Nitrogen simulation models cannot be widely used to evaluate N management practices and irrigation water management practices that increase irrigation efficiencies because of the lack of required input data.

Irrigation efficiency is one of the important factors affecting river basin water quality. Questions frequently arise as to whether increasing irrigation efficiency in a basin will have a net positive or a net negative effect on basin water quality. Quality and quantity of return flow from irrigation are related to irrigation efficiencies and leaching fractions. River basins have leaching fractions analogous to irrigation leaching fractions inasmuch as salts generated by mineral weathering and those dissolved by percolating waters are carried out of the basin by a fraction of the precipitation falling on the basin. Reducing the leaching fraction of a basin by increasing irrigation efficiency and consuming more water in the basin will decrease downstream water quality and quantity. Irrigation efficiency, as a salinity control measure, must be considered in the context of total water management to be properly interpreted.

Flow regulators in both irrigation and storm water engineering normally must not be very exact, but should be simple to construct and maintain, “fool‐proof,” and of course inexpensive. A new flow regulator which matches these requirements, the so‐called Automatic Throttle Hose, is described. The device delivers an almost constant discharge independently of fluctuations in the upstream water level. No external power supply is needed for running the device, as it takes all required energy from the fluid itself. Following an explanation of the regulating principle, the application of this new regulator to irrigation practice is described and a design chart for application in the low head range presented. Further, modifications are listed which may be used to extend the effective head range. Practical experiences in both irrigation schemes and storm water projects indicate that this flow regulator has a place alongside other proven regulators for rather low discharges.

The Senegal River is a major natural resource in West Africa where the principal economic resources are agricultural. A proposed irrigation project will provide a significant increase in crop production and will exert a large influence on the economics of Senegal, Mauritania, and Mali. The magnitude of benefits from the project will depend upon the allocation, scheduling and managing of that portion of the water to be used for irrigating agricultural crops. A procedure is recommended for estimating crop water requirements that only requires the measurement of maximum and minimum temperatures. This procedure although calibrated for the Senegal River Basin using climatic data from four representative locations appears to be generally applicable for other areas without calibration. The importance of rainfall in supplying part of crop water requirements is described. Mean, actual dependable and effective precipitation values are compared for one location. Block farming or the planting of a single crop to manageable areas should be made mandatory. Project managers must assume responsibility for implementing procedures to insure good productivity and reasonable efficiencies.

A mathematical model, which is founded on the kinematic‐wave and Green‐Ampt equations, is presented for the conjunctive overland flow‐infiltration process. Within the context of this model, it is shown that overland flow resulting from a specified time distribution of rainfall intensity can be described in terms of only three physically‐based nondimensional parameters. In cases where a less precise geometric shape is adequate, overland flow hydrographs for the 24‐hr SCS‐Type II rainfall can be approximated by triangular hydrographs. It is possible to evaluate the elements of such a triangular hydrograph in terms of the three governing physically‐based parameters of the overland flow‐infiltration process without using a computer. A dimensionless standardized triangular hydrograph is proposed that predicts the peak discharge, the time of occurrence of the peak discharge, and the volume of runoff before and after the hydrograph peak.

Since 1975, the Bureau of Reclamation has installed approximately 25.8 miles (41.5 km) of PVC (polyvinyl‐chloride) lining in the distribution system of the Riverton Irrigation Project. The lining is part of a $40,000,000 rehabilitation program designed to stop waterlogging of adjacent farmland and conserve water. This paper describes the methods used in investigating, designing, and constructing the PVC lining. It also summarizes the construction costs from 1973 to 1982.