Watershed storage-baseflow relations and monthly water balances at Panola Mountain Research Watershed, Georgia, U.S.A

Watershed storage is a significant part of a catchment water budget, especially at smaller time scales, but is difficult to measure or estimate. A watershed storage-baseflow relation was developed by combining a stream baseflow-recession analysis with a watershed water budget during the dormant season where calculated potential evapotranspiration (PET) was similar to actual evapotranspiration (ET). The relation was developed during baseflow periods such that transient storage, as occurs during hydrologic events, was minimized. The analysis was applied to the Panola Mountain Research Watershed, a small 0.41-hectare forested watershed near Atlanta, Georgia, U.S.A., and resulted in a highly significant relation (R²=0.92, p<0.0001). The watershed storage-baseflow relation was then used to estimate changes in storage during baseflow conditions on a monthly basis. Baseflow storage ranged by 430 mm over the 22-year study period, water years 1986-2007, with an average interannual range of 247 mm. Baseflow and storage peaked at the beginning of April and was at its lowest at the beginning of September. Monthly storage increased an average of 27 mm/month during the dormant season (November - February) and declined by an average of 14 mm/month during the growing season (April - September). The greatest declines in storage were calculated for April and May, averaging 39 and 44 mm/month, respectively. Theses declines were the result of high baseflow runoff in addition to increased ET at the onset of the growing season. The incorporation of storage into annual water budgets modified calculated water yields, changing the range of annual yields from 16-50% to 9.7-46%. Since changes in baseflow storage are now estimated, ET can be calculated by difference using the water budget equation (ignoring transient storage changes). ET averaged about 40 mm/month during the dormant season and 88 mm/month during the growing season and peaked with an average of 123 mm/month in July. ET averaged about 89% of PET during the dormant season and 70% of PET during the growing season. PET represents the potential maximum ET, assuming no water limitations, and was calculated from various meteorological parameters using the Priestley-Taylor equation. Growing season monthly ET, with-respect-to PET, is affected by both low monthly precipitation (defined as less than 50 mm) and low storage conditions (defined as when storage at the beginning of a month is within the lowest 120 mm of storage observed during the study period). Monthly ET was low for months with low monthly precipitation irrespective of storage conditions, with ET 44% of PET (n=6) for low storage conditions and 46% of PET (n=23) otherwise. For growing season months without low monthly precipitation, low storage differentiated ET more, with low storage months having ET 67% of PET (n=18) compared to months without low storage with ET 79% of PET (n=85). These results indicate that transpiration during the growing season is limited probably by water availability during low storage-low precipitation conditions during the growing season. Low monthly precipitation is a stronger indication of low ET/PET ratios than low storage conditions, and it is likely that soil water derived from recent precipitation is the more important control on ET, with transpiration as the driver.