HYDROTHINK
ET Measurements Require Care
William J. Stone, MEM-2164
Water budget is an important aspect of water-management studies. Precipitation is the ultimate source of water resources. Upon reaching the ground it is distributed in one of three ways: runoff (RO), evapotranspiration (ET) or recharge (R). Therefore, the water-balance equation is commonly general- ized as P = RO + ET + R. If data are available for all but one of these parameters, the value for the unknown one can be backed out, for example, ET = P – (RO + R). But until recently, solving the equation was made difficult because two of the parameters, ET and R, were usually unknown. However, technological advances now facilitate the measurement of ET and a mass-balance method, using soil-water chloride content, gives long-term average recharge rates.
In the case of ET, the rate of passage of a laser beam or sound wave between a transmitter and receiver over a short known distance can be used to determine the water content of the air and ultimately the water loss from plants and soil beneath it. These methods were designed for use on the air over large fields of single agricultural crops. However, the USGS has successfully applied the methods to large, uniform stands of native vegetation as well (for example, greasewood in Nevada).
A failed ET study I learned about provides some pointers on how not to apply the method. A station was reportedly set up along a stream in a fairly deep, narrow canyon where a single species of pine was the main vegetation type. So far so good. But the instruments were placed a few feet above the ground surface rather than above the vegetation canopy, reportedly to target evaporation from the soil. The original method measures that as well as transpiration since it is the total water loss to the atmosphere that is of interest in water-balance studies. Furthermore, instruments were not left in place for regular monitoring, but deployed only when measurements were to be taken. Finally, power was to be provided by solar panels, but these were inexplicably set up on a north-facing slope where they were in the shade much of the time.
As might be expected, few data were generated and, per- haps mercifully, a flood eventually wiped out the short, poorly anchored instrument platform. Fortunately, the expensive instruments were not present at the time. The tall, heavy towers normally used to properly deploy the equipment would have withstood most runoff events. Tip: if a flood wipes out your ET study, you’re probably doing it wrong!
Dr. Stone has more than 30 years of experience in hydro- science and is the author of numerous professional papers as well as the book, Hydrogeology in Practice – a Guide to Characterizing Ground-Water Systems (Prentice Hall). Feel free to argue or agree with him by email (wstone04@gmail. com).
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