About Precipitation and Lake Evaporation Data for Texas

Daily or monthly precipitation and pan evaporation measured data are retrieved from: (1) Texas Water Development Board (TWDB) and NOAA's National Weather Service (NWS) evaporation stations; (2) Hydrosphere National Climatic Data Center (NCDC), Summary of the Day Compact Disc; (3) NCDC daily or monthly climatological data for Texas and its surrounding states: Louisiana, Arkansas, Oklahoma, and New Mexico; and (4) Other internet data sources, for processing by methodology discussed below.

Disregard the format of raw data collected, staff from TWDB prepares monthly and annual dataset for all stations for further computation on year by year basis by a geographic information system based program called ThEvap, developed by using ARC Macro Language (AML) by TWDB in 1998.  ThEvap not only computes pan evaporation for an area of Thiessen polygon it made surrounding a station, but also converts Thiessen polygon data to quadrangle data by area-weighted average over intersected Thiessen polygons by a quadrangle.  It then converts pan evaporation rate to reservoir surface evaporation rate for each quadrangle by applying the updated evaporation pan-to-lake coefficients.  In order to collect all available data for the previous year, we usually process the data annually for the previous year in May of current year.

ThEvap program replaced an older program, WD0300, previously ran by the Texas Department of Water Resources.  It has been used to recalculate gross lake evaporation rates between 1954 and 2004.  Data since 2005 were done directly by ThEvap.  Lake evaporation computed from data prior to 1954 is still only available as output from WD0300.  Prior to 1954, WD0300 used pan evaporation data from non-standard pans, which allowed for a much larger dataset.  However, updated pan-to-lake coefficients were not based on non-standard pans, so the decision was made to keep the WD0300 data from 1940 through 1953, but to separate it from the ThEvap dataset since 1954 (for 1940-1953 evaporation).

TWDB also ran ThEvap to produce average monthly precipitation by quadrangle from 1941 to 2011.

The pan-to-lake coefficients used in ThEvap were developed in response to the National Weather Service's NOAA Technical Report NWS 33 in 1982.  They reflect both a seasonal and spatial distribution, instead of the previous annual coefficient (TWDB, Report 64, 1967).  The new coefficients resulted in reduced annual evaporation rates (the new pan-to-lake coefficients table).

For the monthly gross lake evaporation rates, there are some missing data had been replaced by the long-term mean values of quadrangle.

The quadrangle we used here is designated smallest practical area for evaporation calculations in a "square" of 1 degree latitude and 1 degree longitude in size.