Assessment of spatial and temporal variation of potential evapotranspiration estimated by four methods for South Carolina, USA

Abstract

Given the State of South Carolina’s ongoing water planning efforts, in this study we evaluated seasonal and annual potential evapotranspiration (PET) using measured Class A pan evaporation and three widely used estimation methods for the State with three distinct physiographic regions (Coastal, Piedmont, and Mountainous). The methods were temperature-based Hargreaves-Samani (H-S), radiation-based Priestley-Taylor (P-T), and fully process-based Penman-Monteith (P-M). The objectives of the study were to: (a) describe seasonal and temporal distribution of PET by all methods, and (b) quantify differences among PET methods in each region and among regions for each PET method, and (c) identify relationships between monthly pan evaporation (PE) and estimated PET by each method. Daily weather variables from 59 National Oceanic and Atmospheric Administration (NOAA) weather stations distributed in the three regions of South Carolina were used to estimate daily PET for an 18-year period (1998-2015). Net radiation was estimated using modeled solar radiation values for weather station. The average annual H-S PET values for 1998-2015 were 1232± 9, 1202± 11, and 1115± 10 mm for the Coastal, Piedmont, and Mountainous regions, respectively, while the average annual P-T PET values for the same period and regions were 1179 ± 10, 1137± 11, and 1082 ± 11 mm, respectively. Both the mean annual H-S PET and P-T PET for the Mountainous region were significantly (α= 0.05) lower than for the Coastal and Piedmont regions. The mean annual P-T PET for the Coastal region was significantly (α= 0.05) greater than that for the Piedmont. Regional differences showed that estimated PET from 1998-2015 was greatest in Coastal and lowest in Mountainous region. Comparison of all three methods using only data from a common 8-year period showed mean annual P-M PET, varying from 1142 mm in the Piedmont to 1270 mm in the Coastal region, was significantly higher than both the H-S and P-T PET in both regions. The greatest mean monthly H-S and P-T PET values were observed in June and July. Statistical evaluation showed a better agreement of H-S PET with measured pan evaporation followed by the energy balance-based P-T method that used modeled radiation data. However, the latter method may be preferred for PET estimation for South Carolina with a humid climate and also dominated by forest landuse, given more rigorous ground-truthing of modeled solar radiation as data become available. Accordingly, surface interpolation algorithm, Inverse Distance Weighted (IDW), was used to spatially map the distributed P-T PET for the State. Results from this study can be used to support several components of the ongoing water planning efforts in the State of South Carolina.

Citation

Amatya, D.M.; Muwamba,A.; Panda, S.; Callahan, T.J.; Harder,S.; and Pellett, C.A. 2018. Assessment of spatial and temporal variation of potential evapotranspiration estimated by four methods for South Carolina, USA. J. South Carolina Water Resources 5(1):3-24