water and the crop tolerance to salinity dictate the amount of leaching. Irrigation management entails consideration of salinity as well as water content in the root zone.
A canal, well, or pond may serve as the irrigation water supply. A means of transferring the water from the source to the root zone must be designed. Irrigation systems can be broadly characterized as being pressurized or nonpressurized. Pressurized systems are those in which water is delivered through a pipe under pressure and discharged by one of a variety of different outlet designs including sprinkler heads and drip emitters. Nonpressurized systems are those in which water is delivered and allowed to flow across the field. Various configurations for flow across the field such as furrows or basin borders are possible. All irrigation systems deliver water to the soil surface from which it must infiltrate into the soil to recharge the soil-root storage capacity. The infiltration rate therefore becomes an important factor in irrigation management.
Nonpressurized irrigation systems deliver water in a manner that causes free-standing water on the soil surface. The following description of water infiltration into soils applies for the case of free-standing water on the soil surface and is therefore appropriate for nonpressurized irrigation systems.
The infiltration rate decreases with time and approaches a constant steady state rate as depicted in Figure 1. The soil water content at the time of water application affects the initial infiltration rate. The initial infiltration rate is more rapid for a dry as compared to a wetter soil. The infiltration rate is largely controlled by the soil pore size with the most rapid infiltration occurring with soils with large pores. Therefore, different soils have different infiltration rates. Furthermore, the infiltration rate of a given soil can be modified by tillage operations. For example, a soil that is plowed and tilled to produce a seedbed at the beginning of the season has high infiltration rates. However, the process of wetting the soil causes the soil aggregates to disperse and become more compact and thus the infiltration rate for many soils decreases with successive irrigations after tillage. Modification of irrigation management during the growing season to accommodate changes in soil infiltration characteristics is very difficult to program.