Estimation of Evapotranspiration from Fields with and without Cover Crops Using Remote Sensing and in situ Methods

Brett Hankerson,Jeppe Kjaersgaard,Christopher Hay

doi:10.3390/rs4123796

Abstract

Estimation of actual evapotranspiration (ETa) based on remotely sensed imagery is very valuable in agricultural regions where ETa rates can vary greatly from field to field. This research utilizes the image processing model METRIC (Mapping Evapotranspiration at high Resolution with Internalized Calibration) to estimate late season, post-harvest ETa rates from fields with a cover crop planted after a cash crop (in this case, a rye/radish/pea mixture planted after spring wheat). Remotely sensed EToF (unit-less fraction of grass-based reference ET, ETo) maps were generated using Erdas Imagine software for a 260 km2 area in northeastern South Dakota, USA. Meteorological information was obtained from a Bowen-Ratio Energy Balance System (BREBS) located within the image. Nine image dates were used for the growing season, from May through October. Five of those nine were captured during the cover crop season. METRIC was found to successfully differentiate between fields with and without cover crops. In a blind comparison, METRIC compared favorably with the estimated ETa rates found using the BREBS (ETλE), with a difference in total estimated ETa for the cover crop season of 7%.

Highlights

Considerable uncertainty exists regarding the spatial and temporal variability and distribution of the consumptive water use, for agricultural crops, and for rangeland, riparian zones and other areas with natural vegetation
The objective of this research was to examine the utility of using METRIC to estimate cover crop water use and compare the ETa estimates produced by METRIC to ground-based flux point measurements
The cover crops did not reach, let alone maintain, the estimated ETa rates of the previous wheat crops—with cover crop EToF rates peaking around 0.6, while wheat maintained around 1.25 for a month—but they did generate more than enough ETa for METRIC (Mapping Evapotranspiration at high Resolution with Internalized Calibration) to detect and create an EToF curve

Summary

Introduction

Considerable uncertainty exists regarding the spatial and temporal variability and distribution of the consumptive water use, for agricultural crops, and for rangeland, riparian zones and other areas with natural vegetation. The variability is caused by local and regional differences in weather, variations in precipitation distribution and quantity across a region and differences in soil types, land form and land use, vegetation types, cultivar and cropping systems, irrigation application methods and land management. The vast majority of the consumptive water use is made up by evapotranspiration (ET). A common method to estimate actual ET (ETa) from the ground and vegetation includes multiplying a calculated weather-based reference evapotranspiration (ETo) with a crop coefficient (Kc) [1,2]. It is difficult to predict the correct crop growth stages for a large population of crops and fields

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Conclusion