Assessment of Surface Energy Balance Algorithm for land (SEBAL) on estimation of pistachio water requirement in saline condition.

Estimation of crop evapotranspiration under real conditions of on-farm management practices, quality of soil and water resources and environmental stresses have important roles for improving management practices of the region. In the present study, Surface Energy Balance Algorithm for Land (SEBAL) was employed and evaluated for estimating of daily and seasonal actual evapotranspiration (ETa) in Pistachio orchards of northern parts of Ardakan, Yazd Iran, under saline soil and water condition. For this purpose, 24 cloud free MODIS images were acquired and used for estimating of ETa. To evaluate the algorithm on a daily scale, Time Domain Reflectometery (TDR) method was used for daily monitoring of soil moisture (August to September 2012) from 0 to 120 cm depth in 25 sites. To evaluate the algorithm on a seasonal scale, annual released water to the command area (I) and annual deep percolation (DP) was measured to calculate the remaining part of water balance equation (annual ETa) and compared with the SEBAL output. To calculate the DP, the map of leaching fraction for the study area was generated using the Wetting Front Detector (WFD) coupling with electromagnetic induction device (EM38). Afterwards, average of the leaching fraction map (LFavg) was multiplied by released water (I) to calculate annual deep percolation (DP), as well. Results of soil moisture monitoring showed that the trend for its daily changes varies in each site, but the average soil moisture content is relatively constant and ranges from 15 to 20 percent, volumetrically (average is 17.6% and standard deviation is 3.6%). WFD and EM38 integrations for LF calculation also showed that average leaching fraction (LFavg) is about 53%, which is higher than the recommended leaching requirement for the Ardakan pistachios. Chemical analysis of irrigation water and leachate resources revealed that Na and Cl are the major ions of these resources, which refers to high mobility feature of these ions, compared to others (e.g. Ca and Mg). This comprehensive case study led to evaluation of SEBAL outputs in daily and seasonal time scales. For both scales, 7% and 6% of underestimates were resulted for SEBAL ETa, respectively. Also seasonal pistachio actual evapotranspiration was about 1275 mm, while, values of 7 to 8 mm per day was observed maximum water use in August.