13th Conference on Applied Climatology and the 10th Conference on Aviation, Range, and Aerospace Meteorology

Wednesday, 15 May 2002: 9:45 AM
Trends in North Carolina evapotranspiration and links with water availability
Peter J. Robinson, University of North Carolina, Chapel Hill, NC
Poster PDF (187.1 kB)
Water usage throughout North Carolina is increasing and water shortages, more fundamental and long-lasting than the traditional recurring droughts, are beginning to appear. Several counties in the east are under state mandate to reduce their water use to ensure that no further fall in water table level occurs. However, the amounts of water actually available, primarily determined by the atmosphere, are poorly known. The precipitation input is well monitored and understood, but the evapotranspiration output is poorly defined. This paper presents an initial estimate of the annual average statewide outgoing vertical moisture flux. Three approaches were used: actual evapotranspiration using the mass balance difference between precipitation and runoff; potential evapotranspiration using estimates based on empirical formulae, and evaporation using an integration of pan evaporation measurements. The differences in long-term average values between each method were consistent with theory. However, for temporal trends more pertinent to water supply planning, the three methods gave contradictory results. In particular, pan observations indicated a 15% increase in evaporation over the last 50 years, while the mass balance approach produced a 5% decrease in actual evapotranspiration. Re-evaluations of water storage estimates and of pan observations are underway, suggesting a small increase in both actual and pan evaporation. A more detailed spatial analysis algorithm, using the available sporadic soil moisture and atmometer-based evaporation estimates, is being developed. This will provide refined statewide estimates and some preliminary regional moisture-flux values.

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