Development and evaluation of site-specific evapotranspiration models in Malawi through a comparative analysis of existing models

Locally based information on reference evapotranspiration pertinent to efficient irrigation water management is scarce in developing countries. This compromises the accuracy in determining irrigation crop water requirements since most existing reference evapotranspiration models are empirically based and less accurate unless calibrated to local conditions. The FAO Penman-Monteith equation has been proven to estimate reference evapotranspiration for different environments worldwide. However, its intensive data input requirements impede utilization of the model in developing countries due to inadequate climate data collection and management capacities. Therefore, a need existed to develop site-specific models that are less data intensive. A study was conducted aimed at developing site-specific evapotranspiration models using comparative analysis of the Kharrufa, Linacre, Hargreaves Original and Hargreaves Modified empirical models during the wet and dry season. Performance of each model was compared with the FAO Penman-Monteith model, regarded as the standard model. The models whose Root Mean Square Error (RMSE), Mean Bias Error (MBE) and coefficient of determination (R²) were satisfactory were selected. The selected models were developed statistically through regression analysis. Performance of the Kharrufa model was satisfactory comparatively as observed from its RMSEs of 1.02 mm/day and MBE -0.34 to 0.8 for the dry season. However, the model less accurately estimated reference evapotranspiration during the wet season (RMSE 2.18 mm/day and MBE 1.03 to 2.13). The study recommends the use of the Kharrufa model in the study locations in dry seasons while utilization in wet season may require further studies to ascertain the model's useability and reliability.