Evapotranspiration is the water evaporated from the ground back to the atmosphere both as transpiration from the leaves of plants and also as direct evaporation from open water and soil.
The amount of water evaporated changes from day to day based on cloud cover, wind, relative humidity, temperature and other influences. On a sunny day this time of year, an average of approximately one-third of an inch of moisture is evaporated in a day. On a cloudy day that amount may be a tenth of an inch or less while on a hot, sunny day it could be more than half an inch.
This is why our drought is worsening and will likely continue to worsen. Sporadic quarter to half-inch rains simply cannot keep up with evaporation rates. Without any general soaking rains, the ground is losing moisture faster than it is being replenished.