Document Type

Dissertation - Open Access

Award Date


Degree Name

Doctor of Philosophy (PhD)



First Advisor

Maurice L. Horton


Approximately 70% of all precipitation is lost through percolation out of the root zone or by evapotranspiration (evaporation from the soil plus transpiration from plants). Estimation of these water loss processes has generally been on a small scale due to the cost and time required for their measurement. Development of remote thermal scanners has provided a possible means of estimating water loss from surfaces by using the surface temperature. This study was designed (a) to determine water movement patterns in the soil profile and (b) to evaluate the feasibility of using canopy temperatures in estimating evapotranspiration rates from cropped areas. Soil water flux values were estimated using tensiometer data in both the irrigated and nonirrigated areas. Flux was upward in all soil depth intervals in the nonirrigated area during the study. Flux in the 130-150 cm depth interval remained downward throughout the study in irrigated sorghum. The flux in the 15-30, 30-50, and 50-70 cm depth intervals reversed and became upward within one week following the irrigation of sorghum. If flux out of the root zone had been neglected, and profile water depletion equated with evapotranspiration (ET), ET would have been overestimated in the irritated sorghum and underestimated in the nonirrigated sorghum. Canopy temperature data indicated that the nonirrigated canopy was usually 1-3 C warmer than the irrigated canopy during daylight hours; and that during nighttime, there was no clear temperature difference between canopies. On most dates, the air temperature was warmer than canopy temperature, often by as much as 3-5 C. During the hours of 0000 to 0800 CDT, the canopy temperature was usually warmer than air temperature, often by 5-6 C. Tensiometer data yielded smaller estimates of evapotranspiration rates than did five microclimate equations used. The equations are for potential ET or have been derived neglecting energy sinks; therefore, their estimates of ET would have been expected to be larger than tensiometer estimates. Two of the five microclimate equations used (Bartholic and Brown), employ the canopy temperature in estimating LT rates. Bartholic ET estimates were found to be approximately 17% smaller than typical ET estimates by the Penman and energy budget-Bowen ratio methods. The Brown method yielded ET rates approximately 22% larger than typical Penman and energy budget-Bowen ratio estimates of ET rates. The Bartholic method requires ·1ess input data than the Brown method. Therefore, the Bartholic method is slightly more desirable because of accuracy and less input data required.

Library of Congress Subject Headings

Evaporation control
Plants -- Transpiration




South Dakota State University