MAPS OF REGIONAL EVAPOTRANSPIRATION AND RUNOFF PRECIPITATION RATIOS IN THE NORTHEAST UNITED-STATES

Regional maps of evapotranspiration (ET) and runoff/precipitation ratios (R/P) are rare for the United States. We have found mapping of ET and R/P to be a useful part of our work on runoff mapping, which in turn was a necessary part of our assessments of effects of acidic deposition. Maps of ET and R/P could be useful in other water resource analyses as well. Toward this end we present maps of these variables that we have prepared for the northeastern United States. We compute ET and R/P from the water balance equation with measured runoff and precipitation and (in one case) modeled estimates of precipitation. We have mapped ET and R/P in three different ways. First, we have mapped them by Major Land-Use Resource Areas (MLRA). We have performed this mapping for both a long-term period (1951-1980) and an especially wet water year, water-year 1984 (WY84). Second, we have mapped ET and R/P using an automated contouring procedure in association with modeled estimates of precipitation from PRISM (Precipitation-elevation Regressions on Independent Slopes Model). Third, we have manually contoured the variables based on measured values of runoff and precipitation as they appear to relate to elevation, land use, and vegetation. Our maps compare well with each other and with the relatively few prior efforts at such mapping for the region. Our findings indicate that ET tends to be much more conservative over time than R/P, which tended to be greater in the wet year (WY84). As would be expected, the three approaches have relative strengths and weaknesses. Mapping by MLRA may be more useful when measured estimates are few or when lack of local expertise limits contouring subjectively. Contouring allows for more detailed representation of patterns. Automated contouring has the advantages of efficiency and reproducibility, whereas manual contouring allows the exercise of potentially more accurate expert local knowledge, when it exists.