Estimation of Turc reference evapotranspiration with limited data against the Penman-Monteith Formula in Senegal

Estimation of daily evapotranspiration (ET0) is required for efficient irrigation management in Senegal but physically based equations such as the Penman-Monteith (PM) approach need large inputs data which are not always available. Therefore empirical approaches such as Turc's formula are often used. Unfortunately, This latter showed high annual ET0 compared to FAO Penman-Monteith (FAO-PM) ET0 for stations in this region. Moreover, Turc underestimates mean daily ET0 for the dry season and overestimates ET0 for the wet season. For reevaluation, Turc's empirical parameters (a and b) were fitted to match FAO-PM ET0, whereby the fitting did not improve the prediction substantially and only assuming C being independent on relative humidity yielded better agreement. In a second step universal a and b parameters were estimated for different stations in Senegal. The results indicate that one set of parameters (a and b) can be used to predict all station ET0 with fairly high accuracy indicated by a R-2 of 0.80 and an RMSE of 0.71 mm day(-1). Based on these findings we propose to use Turc's approach in Senegal only with the parameters a = 0.16 and b = 29.19 with the constrain that C is independent on relative humidity.