A water mass balance approach was proposed as a design approach for surface application of effluent for on-site wastewater systems, but little field study was available to verify the validity of the approach. The mass balance was investigated using plots at the wastewater treatment plant of the City of Littlefield, Texas, from October 2005 to September 2007. The highest monthly precipitation is 181 mm while the lowest is 1 mm; the individual Christiansen's coefficient of uniformity (UCC) ranged from 31 to 75%, yet the annual UCC was 83%. The average daily leaching amount was measured in the range of 0-2 mm. The ratio of leaching to total water input in each sampling period is 0.49-37.05%, while the ratio of cumulative leaching to cumulative total water input is relatively constant and ranged from 12 to 14%. The regression model between cumulative leaching (y) and cumulative total input (x) was analyzed and resulted in an R 2 of 0.98.