Improved Method of Estimating Crop Consumptive Use and Distribution of Soil Water in Irrigated Soil Profiles

Field-embedded lysimeter tests were conducted to determine soil and plant evaporation from a trickle-irrigated cotton field located in the lower Rio Grande basin of New Mexico. Soil evaporation was determined gravemetrically (1-10 cm) and with embedded containers. Evapotranspiration was determined from changes in soil water content, from the depths of applied irrigation water, and the volume of drainage water. The transpiration and soil evaporation were correlated with leaf area index, soil water, and potential evaporation estimated by Penman equations. Empirical equations describing the effect of these parameters on transpiration and soil evaporation agreed very well with the results of Ritchie, obtained under dryland farming of central Texas. It was also found that transpiration did not decrease below potential level until about 60% of the available soil water was depleted.

A method was developed to determine the distribution of cotton roots with depth and time in field soils. The method consisted of soaking soil and roots removed from the field in a solution containing 40 g/l sodium hexametaphosphate in a 1:5 soil solution ratio. The density of the resulting soil suspension is increased to 1.5 g/cm ³  by added dry 78% pure CaCl ₂  in a ratio of 1 g CaCl ₂  to 2 g soil suspension. Roots float to the surface and are skimmed from the surface with a fine wire strainer. By subsequent washing in tap water, roots and organic debris may be further separated. The method was used to determine the distribution of fine roots (mean diameter 0.25 mm) and total root mass for field-grown cotton at various times and depths. The method was found to be inexpensive and efficient.

A computer simulation model was developed to compute changes in water content with depth and time in a layered soil. The model uses the hydraulic properties of the soil, climatological data, and root distribution as determined in the field. Computed water contents compared well with water contents measured in the embedded lysimeters planted with cotton and irrigated over a 70-day period. The model was further tested by varying the hydraulic conductivity versus water content relationship and the root distribution with depth. These changes appeared to have less effect on predicted water-contents in comparison with observed water distributions than did modifying evapotranspiration rates. It appears that water uptake by cotton roots at any particular point in the soil profile was found to be a function of the root mass at that point and the depth below the soil surface. The results of this study show that water-content profiles in a given soil with an actively growing crop are to a large extent determined by the external evaporation demand rather than by the hydraulic properties of the soil.

Project 3109-208