Using Very High Resolution Thermal Infrared Imagery for More Accurate Determination of the Impact of Land Cover Differences on Evapotranspiration in an Irrigated Agricultural Area

Jie Cheng,William Kustas

doi:10.3390/rs11060613

Abstract

Land cover has a strong effect on the evapotranspiration (ET) and the hydrologic cycle. Urbanization alters the land cover affecting the surface energy balance and ET by, for example, urban encroachment in agricultural areas. This study investigates the potential utility of high resolution ET in determining more accurately the impact of land cover on water use for an agricultural area. The approach was to apply the physically based two-source energy balance (TSEB) model to very high resolution (~8 m) aircraft thermal data and compare the ET pattern and distribution to TSEB output using the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data acquired on 2 August 2012. Modeled flux components were validated using measurements collected from a network of 16 eddy covariance (EC) towers at the study site. The modeled ET using the aircraft data agreed satisfactorily with the flux tower measurements and had better performance than the TSEB model applied to the ASTER data. The percent errors between ET closed by the Bowen ratio (BR) and residual (RE) approaches were 3 and 1%, respectively. It is shown that the high resolution aircraft ET can more accurately determine the change in ET magnitude by having pure pixels of the main land cover types, namely urban, agriculture, and natural vegetation. As a result, the ET histogram exhibits a significant bi-modal distribution which can be used to accurately distinguish the impact on ET from urban versus agricultural land cover areas and potentially monitor the effect on ET over a landscape due to small changes in land cover. At the coarser 90 m resolution of ASTER, the TSEB ET estimates are more often a combination of urban and agricultural land cover ET near the urban-agriculture land cover boundaries. As a result, the bi-modal distribution in ET is almost nonexistent. This study demonstrates the potential utility of high resolution ET mapping for more accurately determining the magnitude of the ET differences between cropland and urban land cover. It also suggests that, with high resolution thermal imagery, TSEB is a potential tool for monitoring the impact on ET due to relatively small changes in land cover as a result of urban expansion. Such a tool would be useful for watershed management.

Highlights

As an essential component of surface energy and water balance, evapotranspiration (ET) is critical to understanding the interactions between the hydrosphere, atmosphere, and biosphere [1,2,3]
The main objective of this study is to investigate the possibility of monitoring the impact of land cover differences on ET using high resolution thermal imagery over agricultural areas
The physically based two-source energy balance (TSEB) model has been proven to be a powerful tool for predicting surface energy balance and in particular, for ET or water use estimation

Summary

Introduction

As an essential component of surface energy and water balance, evapotranspiration (ET) is critical to understanding the interactions between the hydrosphere, atmosphere, and biosphere [1,2,3]. During the past few decades, significant progress has been made on remotely sensed estimates of ET, and numerous remote sensing-based methods have been developed. These approaches fall into five main categories: (1) empirical statistical methods [8,9]; (2) surface energy balance models [10,11,12,13]; (3) the traditional ET estimation approaches combined with remote sensing [14,15]; (4) surface temperature-vegetation index triangle/trapezoid space methods [16,17]; and (5) data assimilation methods [18]

Objectives

Methods

Results

Conclusion