Daily evapotranspiration is a major component of water resources management
plans. In arid ecosystems, the quest for an efficient water budget is always
hard to achieve due to insufficient irrigational water and high
evapotranspiration rates. Therefore, monitoring of daily evapotranspiration
is a key practice for sustainable water resources management,
especially in arid environments. Remote sensing techniques offered a great
help to estimate the daily evapotranspiration on a regional scale. Existing
open-source algorithms proved to estimate daily evapotranspiration comprehensively in arid
environments. The only deficiency of these algorithms is the
course scale of the used remote sensing data. Consequently, the adequate
downscaling algorithm is a compulsory step to rationalize an effective water
resources management plan. Daily evapotranspiration was estimated fairly well
using an Advance Along-Track Scanner Radiometer (AATSR) in conjunction with
(MEdium Resolution Imaging Spectrometer) MERIS data acquired in July 2013 with
1 km spatial resolution and 3 days of temporal resolution under a surface energy balance system
(SEBS) model. Results were validated against reference evapotranspiration ground
truth values using standardized Penman–Monteith method with

Evapotranspiration is the principle process in defining mass and energy relationship in the surrounding hydrosphere (Allen et al., 2007a, b; Cruz-Blanco et al., 2014). The consumptive use of irrigational water in agriculture is the fundamental component of a balanced estimation of evapotranspiration (Bastiaanssen et al., 2011; Cammalleri and Ciraolo, 2013).

The concept of water use efficacy basically depends on the reliable estimation of evapotranspiration and surface water evaporation (Berengena and Gavilán, 2005; Elhag et al., 2011). Weather and wind conditions induce a regional and seasonal variation of evapotranspiration estimation (Hanson, 1991; Cristóbal and Anderson, 2013).

Conventional techniques of field scale evapotranspiration estimations are achieved, especially over homogenous surfaces using ordinary techniques: lysimeter systems, eddy covariance (EC) and Bowen ratio (BR). Nevertheless, conventional methods of evapotranspiration estimations are incapable of fulfilling the quest for regional evapotranspiration estimation, specifically in harsh climatic conditions (Gavilán et al., 2006; Ghilain et al., 2011). Therefore, remote sensing evapotranspiration models are adequate techniques for obtaining satisfactory estimates (Allen et al., 2007a, b; De Bruin et al., 2010).

Remote sensing evapotranspiration models are numerous. Several algorithms are already in practice with different complexity levels to estimate evapotranspiration based on different climatic conditions and land use variability (Elhag et al., 2011; Espadafor et al., 2011; Cristóbal and Anderson, 2013).

Based on several scholarly works by Roerink et al. (2000), Su (2002), Crago and Crowley (2005), Chávez et al. (2005), Loheide and Gorelick (2005), Allen et al. (2007a, b), Ghilain et al. (2011), Psilovikos and Elhag (2013) on remote sensing evapotranspiration-based algorithms, there are principally two types of evapotranspiration estimation concepts on terrestrial surfaces.

The first concept is to use the surface reflectance in different visible
(VIS), near-infrared (NIR) and even extended to thermal infrared (TIR)
portions of the electromagnetic spectrum to rationalize the surface energy
balance (SEB). The other concept it to use vegetation indices derived from
canopy reflectance to conceptualize remotely sensed crop coefficient
(

Ground truth data collection exercised at less than 1 m canopy height, at which all related surfaces fluxes and atmospheric surface variables of the vegetation cover takes place in an arid environment (Beljaars and Holtslag, 1991; Zwart and Bastiaanssen, 2004). Based on Brutsaert (1991, 1999), Monin-Obukhov similarity (MOS) and bulk atmospheric boundary layer (ABL) functions were calculated. Brutsaert (1999) suggested sets of criteria to estimate MOS or ABL if scaled down appropriately for a given circumstances. Brutsaert criteria are valid only for unstable conditions.

Therefore, van den Hurk and Holtslag (1995) adjusted and validated Brutsaert
criteria using atmospheric surface layer scaling according to
Brutsaert (1982) to be used in stable conditions. Generic estimation of
surface albedo for vegetated land covers is based on the red (

The aim of the current study is to monitor turbulent heat fluxes in Wadi ad-Dawasir to estimate the daily evapotranspiration rate and relative evaporation ratio using Advance Along-Track Scanner Radiometer (AATSR) and MEdium Resolution Imaging Spectrometer (MERIS) sensors. The final step is to identify the regression coefficient between the estimated evapotranspiration's rates and the actual ground truth data.

Location of the study area (Elhag, 2016).

The study area, the town of Wadi ad-Dawasir, is located on the plateau of Najd at Lat
44

The current research work is based on assessing a regression correlation between estimated evapotranspiration data conducted from AATSR and MERIS sensors and its corresponding ground truth evapotranspiration data conducted through standardized Penman–Monteith. Therefore, accurate synchronization of remote sensing data bypassing and ground truth data collection was exercised.

Remote sensing data were acquired from AATSR and MERIS sensors on the 8th of July 2013. The satellite data were georeferenced to WGS-84 datum, atmospherically corrected using the Simplified Model for Atmospheric Correction (SMAC) (Rahman and Dedieu, 1994). Several meteorological data were collected from a stationary station located within the designated study area (2004–2014, average meteorological data).

The surface energy balance system (SEBS) was initiated by Su (2002) based on
further surface energy balance index improvements. SEBS dynamicity works for
regional and local evapotranspiration (ET) estimation. Regional ET estimation
uses Monin–Obukhov similarity (MOS), bulk atmospheric similarity and thermal
roughness principles. On the other hand, local ET estimation uses only
atmospheric surface layer (ASL) scaling fundamentals (Brutsaert, 1999; Su,
2001; Su et al., 2001). The boundary conditions (wet and dry) are essential
components in ET estimation using the SEBS model. According to the water
availability limitation,

Consequently, an evaporative fraction (

Actual daily evapotranspiration thematic map.

Normal distribution of actual daily evapotranspiration data.

Relative evaporation thematic map.

Normal distribution of relative evaporation data.

The relationship between actual and simulated daily evapotranspiration.

Using a standardized Penman–Monteith method, 50 ground truths data
were collected and used to validate the implemented model. The sampling
locations were consistently distributed over the designated study area. The
lysimeter technique for the estimation of daily evapotranspiration was
carried out following Liu and Wang (1999) with calibrated accuracy equal to

The corrected Penman equations for estimating the daily evapotranspiration
was conducted according to Jensen et al. (1995):

Consequently, the wind function was conducted following to Doorenbos and
Pruitt (1977) as

Meanwhile,

Linear regression model was used to find the correlation coefficient between
the estimated and the actual evapotranspiration values. Root mean square
error (RMSE) was used to signify the inequality of variance and correlation
of the linear regression model (Box, 1954). The RMSE was calculated as follows:

The SEBS model implementation over the designated study area results in 10
different turbulent heat fluxes thematic maps. The histogram and the scatter
plot of SEBS output thematic maps were plotted against the daily
evapotranspiration values. The estimated daily evapotranspiration values
ranged from zero to 6.61 mm day

Implementation of the SEBS model over the designated study area showed higher daily evapotranspiration values than projected. Higher daily evapotranspiration values were noticed because the sensible heat flux is the major part of the energy, while the latent heat flux is dominating only over the agricultural area (Frey et al., 2011; Elhag, 2014a, b). The behaviour of the SEBS model could be explained by its tendency to simulate the potential daily evapotranspiration rather than the actual daily evapotranspiration, which is identified as the lack of leaf area index value over desert areas (Li et al., 2009; Elhag et al., 2011). The application of the SEBS model over the designated study area showed an insignificant difference to the Nile Delta case in term of accuracy assessment (Elhag et al., 2013).

Projected evapotranspiration data using a surface energy balance system model and multiple remote sensing imageries demonstrated robust association with the ground truth data. The application of the surface energy balance system model mapped the daily evapotranspiration and evaporative fraction objectively over Wadi ad-Dawasir region. The findings of the current research will help the decision makers towards a modification of the agriculture activities in similar areas in terms of conservative irrigational water regulations. The model shows consistent results in the estimation of daily evapotranspiration in Nile Delta region and in Wadi ad-Dawasir. Accordingly, the surface energy balance system model can be considered a reliable and effective tool in the estimation of daily evapotranspiration, explicitly in arid environments.

The authors declare that they have no conflict of interest.

This project was funded by the Deanship of Scientific Research (DSR) at King Abdulaziz University, Jeddah, under grant no. (G-182-155-37). The authors, therefore, acknowledge DSR technical and financial support with thanks. Edited by: L. Eppelbaum Reviewed by: N. Yilmaz and S. Boteva