A modified form of Lettau's evapoclimatonomy model is used to calculate the model response functions for runoff, soil moisture, change of soil moisture with time, and evapotranspiration. The model is implemented for the state of Kansas with forcing functions of monthly mean net radiation and precipitation, averaged for 1968–77. The objectives are (i) to implement and test the model on spatial and temporal scales for which it was originally designed; (ii) to test the validity of the model on reduced scales (Part II). If the evapoclimatonomy approach is applicable on shorter time scales, the model output of monthly mean evapotranspiration can be used as independent “ground truth” for testing current point parameterizations to calculate the surface energy and water budgets. This is presently of particular interest since some of the input parameters can be derived from spatially integrated satellite observations.