Introduction and Objective

Evapotranspiration, the transfer of water from its liquid state on the surface of the earth to its gas state in the atmosphere, is a key component of the hydrologic cycle. It is the combination of evaporation, which is the vaporization of water directly from the Earth’s surface, and transpiration, the process by which vegetation releases water to the atmosphere via photosynthesis. Evapotranspiration is a key component of the water budget equation, which is used to balance the amount of water entering an environment to the amount exiting. The water budget equation is as follows: P = Q + ET + ΔS, where P = precipitation, Q = discharge in rivers or surface runoff, ET = evapotranspiration and ΔS = change in storage. While there are many methods to directly measure precipitation, discharge, and change in storage in the reservoirs or aquifers, there is no way to directly measure evapotranspiration; it must be calculated from various environmental parameters. This study compares the evapotranspiration (ET) values derived using two of those methods: measuring ET using data from weather stations as inputs into the Kimberly-Penman equation, and measuring ET using MODIS data from Landsat 8 as inputs into the Penman-Monteith formula. The details of each of these methods are described in Appendix A.