Estimating Actual Evapotranspiration over Croplands Using Vegetation Index Methods and Dynamic Harvested Area

Neda Abbasi,Christian Opp,Pamela Nagler,Stefan Siebert,Hamidreza Salemi,Armando Barreto-Muñoz,Sattar Chavoshi Borujeni,Kamel Didan,Hamideh Nouri

doi:10.3390/rs13245167

Abstract

Advances in estimating actual evapotranspiration (ETa) with remote sensing (RS) have contributed to improving hydrological, agricultural, and climatological studies. In this study, we evaluated the applicability of Vegetation-Index (VI) -based ETa (ET-VI) for mapping and monitoring drought in arid agricultural systems in a region where a lack of ground data hampers ETa work. To map ETa (2000–2019), ET-VIs were translated and localized using Landsat-derived 3- and 2-band Enhanced Vegetation Indices (EVI and EVI2) over croplands in the Zayandehrud River Basin (ZRB) in Iran. Since EVI and EVI2 were optimized for the MODerate Imaging Spectroradiometer (MODIS), using these VIs with Landsat sensors required a cross-sensor transformation to allow for their use in the ET-VI algorithm. The before- and after- impact of applying these empirical translation methods on the ETa estimations was examined. We also compared the effect of cropping patterns’ interannual change on the annual ETa rate using the maximum Normalized Difference Vegetation Index (NDVI) time series. The performance of the different ET-VIs products was then evaluated. Our results show that ETa estimates agreed well with each other and are all suitable to monitor ETa in the ZRB. Compared to ETc values, ETa estimations from MODIS-based continuity corrected Landsat-EVI (EVI2) (EVIMccL and EVI2MccL) performed slightly better across croplands than those of Landsat-EVI (EVI2) without transformation. The analysis of harvested areas and ET-VIs anomalies revealed a decline in the extent of cultivated areas and a loss of corresponding water resources downstream. The findings show the importance of continuity correction across sensors when using empirical algorithms designed and optimized for specific sensors. Our comprehensive ETa estimation of agricultural water use at 30 m spatial resolution provides an inexpensive monitoring tool for cropping areas and their water consumption.

Highlights

Evapotranspiration (ET) is the water flux to the atmosphere through evaporation (E)from the soil and transpiration (T) from plants
Calculated VIs were compared on two dates, 14 June 2002 and 1 August 2008, in four combinations (EVIccL –EVI2ccL, EVIMccL –EVI2MccL, EVIMccL –EVIccL, and EVI2MccL –EVI2ccL )
The highest and lowest estimated values belonged to EVIccL and EVI2MccL, respectively, as evidenced by histograms

Summary

Introduction

Evapotranspiration (ET) is the water flux to the atmosphere through evaporation (E)from the soil and transpiration (T) from plants. LST models like SSEBOp [14] and SEBAL [19] can predict ETa in semi-arid and arid regions where croplands are irrigated. They have limitations owing to the need for parameterization and calibration for each image, their degree of complexity, their sensitivity when defining the wet and dry conditions (cold and hot pixels) in each scene [18], and only a few sensors offer open-source thermal data [20]. VIs provide an integrated measurement of the growing conditions and physiological processes [22] They are useful in regions where water is the main limitation. VI-based methods have not been applied to croplands in large irrigation districts

Methods

Results

Discussion

Conclusion