Adjustment of Thornthwaite equation for estimating evapotranspiration in Vojvodina

Evapotranspiration is one of the crucial components of hydrological cycle. The Penman-Monteith method (PM) is recommended as the sole standard method for estimating reference evapotranspiration (ET₀). The usage of the PM method is limited in many regions due to the lack of required weather data. In such circumstances, simple Thornthwaite equation is often used to estimate ET₀. The main objectives of the present study are (i) to estimate reference evapotranspiration using different Thornthwaite approaches, (ii) to develop optimal adjusted equation, and (iii) to consider the spatial variability of the empirical coefficient(s) of adjusted equation for the study area. In this study, six Thornthwaite approaches were compared to the full set PM equation using weather data from Vojvodina region, Serbia. The original Thornthwaite equation was very poor in estimating ET₀ and greatly underestimated PM values at all locations. It can be concluded that an adjustment of the Thornthwaite equation is necessary. The obtained results indicate that ET₀ could be estimated from the new Th65 approach (effective temperature, k = 0.65), which reproduced statistical characteristics better compared to other Thornthwaite approaches. The spatial variability of the empirical k coefficient showed that k values varied from 0.62 to 0.69 across the study area with deviations of − 5% to 6% compared to a unique k value of 0.65. These results suggested that single regional k value can be successfully used for estimating ET₀.