"Penman-Monteith formulation for direct estimation of maize evapotranspiration in well watered conditions with full canopy"; Hector Flores, Advisor: Dr. Ken Hubbard. An efficient use of water resources for irrigation requires an accurate estimation of crop water requirements. The traditional approach to estimate crop evapotranspiration (ET) requires the use of a crop coefficient and an estimate of a reference ET. However, crop coefficients depend on crop type, management, and weather conditions and they sometimes lead to large errors in crop ET estimates. The Penman-Monteith (PM) formulation can be used to estimate crop ET without using a crop coefficient if the crop surface and aerodynamic resistances are known. PM formulation application also requires weather measurements from above the crop in question; however, this is not possible in everyday applications. This study analyzed the feasibility of using the PM formulation in maize ET estimation and characterized the uncertainties introduced when the weather data are measured above grass. The results show that the PM formulation can be used to estimate the maize ET. The approach worked reasonably well for a full crop cover under well watered conditions. The critical point was to estimate the surface resistance as a function of climatic variables. Maize ET was estimated with values that varied between 0.3 and 0.78 mm/day when the ET was estimated using weather data measured above the maize canopy. The uncertainty introduced when the maize ET was estimated using weather data measured above the grass was relatively small, the increased between 0.12 and 0.2 mm/day.