Application of remote sensing in estimation of actual evapotranspiration for assessment of water requirement, case study

Estimating evapotranspiration (ET) is one of the important parameters in hydrological balance, planning
and water resources management. Hence estimating ET is essential in the scales regarding temporal and
spatial. Although calculating ET is precise in small field scales and using lysimeters, but regarding
locations, such methods have point estimation limitations. Thus, using remote sensing technology could
be an appropriate approach for responding to the mentioned limitation, due to the possibility of places
data estimations.
One of the most considerable models in this respect could be SEBAL algorithm. The aim of this study is
obtaining the actual evapotranspiration (ETa), by using SEBAL algorithm, during 2008 in Ben-Saman
district located in Gavkhouni drainage basin. Despite the conflicts between the two provinces of Esfahan
and Chaharmahal and Bakhtiari, Ben-Saman district that is situated in the downstream of Zayandehroud
Dam with a part of it in Esfahan province and the other part in Chaharmahal and Bakhtiari province, is a
quite sensitive region. Therefore, accessing the water and relevant calculations could be the solution to
the problem. Thus, the time series of MODIS images from Terra satellite were prepared and ETa of the
region were estimated for the year 2008.
To evaluate SEBAL algorithm performance, the algorithm was executed on Nekooabad agricultural
network located in Nafafabad region and three different methods were used to verify the results. 1- Using
Penman-Monteith method: Due to inaccessibility of enough data in the studying district to evaluate the
results obtained from the calculation of annual Eta, the Penman-Monteith method is used for result
comparision. It indicates a correlation of 90.47% of ETa obtained from SEBAL algorithm and the
reference ET obtained by Penman-Monteith method. 2) ET-pan method (Orang method): Due to
inaccessibility of the mentioned tools in the studying district, a comparative method was used for the
obtained results, from ET and using an ET-pan and Ourang equation, to evaluate the results of the
calculations regarding annual ETa. It indicated a correlation of 86.72% of the obtained ETa by SEBAL
method and the reference ET rate from Orang method. 3) Water balance method: by putting the rate of
extracting from the well in the district, rate of percipitation and inflow and outflow in the water balance,
the ET parameter was calculated as the unknown parameter and the value was compared with the result of
SEBAL algorithm. It showed a relative error of 7.21 percent that is an acceptable rate. SEBAL method
has estimated the ETa in Nekooabad agricultural network to be 1143 mm and 228.16 mm in Ben-Saman
region.

Application of remote sensing in estimation of actual evapotranspiration for assessment of water requirement, case study Keywords:

Remot Sensing , Actual Evapotranspiration , SEBAL , Water Balance , ET Pan , Penman- Monteith