DEVELOPMENT OF A WATER BUDGET IN A TROPICAL SETTING ACCOUNTING FOR MOUNTAIN FRONT RECHARGE: TUTUILA, AMERICAN SAMOAAbstract
A threshold-style water budget for Tutuila, American Samoa was developed to estimate mean daily recharge over the west-central part of the island. The study focused on the Tafuna and Leone Plains, areas of low relief and deep soils that lie at the base of a steep mountain range. Mountain front recharge was observed occurring at the mountain/plain interface of the Tafuna Plain and was therefore included as a component part of the water budget.
The Tutuila budget was processed for the 30-year period of 1971 – 2000. The results indicate that, on average, the west-central area of the island receives an estimated recharge of 106 million gallons per day (Mgal/d). Mountain front recharge is estimated to contribute about 8 Mgal/d, or about 8 percent of total recharge. At the mountain/plain interface this percentage increases substantially, in places accounting for more than 50 percent of estimated mean daily recharge.
Different conditions were tested through sensitivity analyses, and generated recharge estimates in the range of 79 – 133 Mgal/d. Changes of plus or minus 15 percent to rainfall (the average annual variability for the period of record) generated changes to estimated recharge of about plus or minus 25 percent. Sensitivity of runoff-to-rainfall ratios was tested by using the minimum or maximum monthly basin value within a region rather than the regional mean monthly value. Using minimum values increases estimated recharge by 12 percent; using maximum values decreases estimated recharge by 14 percent. Use of alternative canopy interception values (canopy interception rate, free throughfall rate, and canopy storage limit) generated changes to estimates of recharge of plus 3 to minus 10 percent. Changes to other parameters generated changes to estimates of recharge no greater than plus or minus 5 percent relative to baseline results.
Sensitivity analyses showed little change in estimates of actual evapotranspiration, with no test producing a change larger than 9 percent, and most frequently less than 5 percent. This may be due to patterns of rainfall that provide frequent and abundant input to the canopy and plant/soil systems at rates that exceed potential evapotranspiration. This finding may have negative implications if a trend develops towards locally drier conditions.
Estimates of recharge for the west-central area of Tutuila, American Samoa were highest where MFR augmented normal inputs. Many of the water-supply wells are located downgradient on the Tafuna Plain downgradient of the mountain/plain interface.