Comparing Remotely-Sensed Surface Energy Balance Evapotranspiration Estimates in Heterogeneous and Data-Limited Regions: A Case Study of Tanzania’s Kilombero Valley

William Senkondo,Subira E Munishi,Joel Nobert,Steve W Lyon,Madaka Tumbo

doi:10.3390/rs11111289

William Senkondo, Subira E Munishi + Show 3 more

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https://doi.org/10.3390/rs11111289

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Abstract

Evapotranspiration (ET) plays a crucial role in integrated water resources planning, development and management, especially in tropical and arid regions. Determining ET is not straightforward due to the heterogeneity and complexity found in real-world hydrological basins. This situation is often compounded in regions with limited hydro-meteorological data that are facing rapid development of irrigated agriculture. Remote sensing (RS) techniques have proven useful in this regard. In this study, we compared the daily actual ET estimates derived from 3 remotely-sensed surface energy balance (SEB) models, namely, the Surface Energy Balance Algorithm for Land (SEBAL) model, the Operational Simplified Surface Energy Balance (SSEBop) model, and the Simplified Surface Balance Index (S-SEBI) model. These products were generated using the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite imagery for a total of 44 satellite overpasses in 2005, 2010, and 2015 in the heterogeneous, highly-utilized, rapidly-developing and data-limited Kilombero Valley (KV) river basin in Tanzania, eastern Africa. Our results revealed that the SEBAL model had a relatively high ET compared to other models and the SSEBop model had relatively low ET compared to the other models. In addition, we found that the S-SEBI model had a statistically similar ET as the ensemble mean of all models. Further comparison of SEB models’ ET estimates across different land cover classes and different spatial scales revealed that almost all models’ ET estimates were statistically comparable (based on the Wilcoxon’s test and the Levene’s test at a 95% confidence level), which implies fidelity between and reliability of the ET estimates. Moreover, all SEB models managed to capture the two spatially-distinct ET regimes in KV: the stable/permanent ET regime on the mountainous parts of the KV and the seasonally varied ET over the floodplain which contains a Ramsar site (Kilombero Valley Floodplain). Our results have the potential to be used in hydrological modelling to explore and develop integrated water resources management in the valley. We believe that our approach can be applied elsewhere in the world especially where observed meteorological variables are limited.

Highlights

Evapotranspiration (ET) is the term used to represent the combined flux from two different pathways of water vaporization in environmental systems, namely, abiotic water evaporation from open water bodies, soil pores, and surfaces of leaves, and biotic leaf transpiration due to diffusion of water molecules from leaf chloroplasts to the atmosphere through stomata [1]
These results suggest that all surface energy balance (SEB) models have managed to capture the actual ET ranges over these land cover classes
Our results show that all the SEB models showed similar patterns in the mean daily ET estimates across all land cover classes

Summary

Introduction

Evapotranspiration (ET) is the term used to represent the combined flux from two different pathways of water vaporization in environmental systems, namely, abiotic water evaporation from open water bodies (e.g., ocean, lakes, and swamps), soil pores, and surfaces of leaves (cuticle), and biotic leaf transpiration due to diffusion of water molecules from leaf chloroplasts to the atmosphere through stomata [1]. Traditional methods (in-situ measurements) used to estimate ET include: weighing Lysimeters, Eddy Covariance (EC) systems, evaporation pans, and Bowen ratio techniques All these methods provide point estimates which are mainly applicable (reliable/representative) at a field/local scale [11,12,13], and their application beyond this scale (such as the river basin scale) requires a good number of measurements (e.g., many weighing lysimeters). Obtaining several measurements is in most cases prohibitively expensive, especially in developing countries typically found in the so-called Global South These traditional ET estimation methods become (practically) inapplicable when resource management scales get large (such as regional or basin-wide scales), mainly due to the heterogeneity of the land surfaces, and the complex nature of heat transfer processes governing the ET [14]

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