Estimating basin scale evapotranspiration (ET) by water balance and remote sensing methods

Abstract

Abstract:Evapotranspiration (ET) is an important hydrological process that can be studied and estimated at multiple spatial scales ranging from a leaf to a river basin. We present a review of methods in estimating basin scale ET and its applications in understanding basin water balance dynamics. The review focuses on two aspects of ET: (i) how the basin scale water balance approach is used to estimate ET; and (ii) how 'direct' measurement and modelling approaches are used to estimate basin scale ET. Obviously, the basin water balance-based ET requires the availability of good precipitation and discharge data to calculate ET as a residual on longer time scales (annual) where net storage changes are assumed to be negligible. ET estimated from such a basin water balance principle is generally used for validating the performance of ET models. On the other hand, many of the direct estimation methods involve the use of remotely sensed data to estimate spatially explicit ET and use basin-wide averaging to estimate basin scale ET. The direct methods can be grouped into soil moisture balance modelling, satellite-based vegetation index methods, and methods based on satellite land surface temperature measurements that convert potential ET into actual ET using a proportionality relationship. The review also includes the use of complementary ET estimation principles for large area applications. The review identifies the need to compare and evaluate the different ET approaches using standard data sets in basins covering different hydro-climatic regions of the world.

Calibration of Spatially Distributed Hydrological Processes and Model Parameters in SWAT Using Remote Sensing Data and an Auto-Calibration Procedure: A Case Study in a Vietnamese River Basin

“…Some local differences occur unavoidably due to the inaccuracy of the various mathematical expressions used to compute a complex hydrological process such as ET. Remote sensing ET values reflect more the real world conditions as they are based on observations [33][34][35]42]. The agreement between SWAT and remote sensing data was expressed by means of the correlation coefficient and the bias.…”

“…Another energy balance method is the Operational Simplified Surface Energy Balance (SSEBop, USGS EROS Center, Sioux Falls, SD, USA) [35] that employs a relationship between ET 0 and a land surface temperature-based scalar (ETrf) to express land wetness. An operational version of SSEB was proposed [78,79] by assimilating air temperature to account for the topographical and latitudinal heterogeneity impact on surface temperature.…”

“…The SSEBop model defines the temperature scalar using hot and cold reference values for any pixel. The cold reference value is estimated as an empirically established fraction of the daily maximum air temperature; the hot reference value is obtained by adding a vertical temperature difference (dT) to the cold reference value [35].…”

“…These uncertainties come in various forms, such as from temperature and net radiation [35], and the aerodynamic component [81]. An uncertainty of up to 4 to 18% can be linked to sources of net radiation [35].…”

Hydroclimatological influences on recently increased droughts in China's largest freshwater lake

“…The lake is located at the south bank of the Yangtze River, which is a humid monsoon climatic region. Although the region historically experiences significant floods (Shankman and Liang, 2003;Shankman et al, 2006), severe lake droughts have occurred frequently in the past decade, resulting in tremendous hydrological, biological, ecological and economic consequences Environment News Service, 2012;Wu and Liu, 2014). Because the lake is the primary part of the well-known Poyang Lake wetland and the lake region serves as an important food base for China, the frequently occurring lake droughts have also received increasing international attention (Jiao, 2009;Finlayson et al, 2010;Liu et al, 2011;Environment News Service, 2012;The Ramsar Convention, 2012;Zhang et al, 2012Zhang et al, , 2014.…”