The difficulty in measuring and/or estimating evapotranspiration at a specific site discussed in Chapter 4 is compounded in developing large-scale evapotranspiration estimates. Nevertheless, spatial evapotranspiration estimates provide a comparable basis for assessing the hydroclimatic implications of energy and moisture relationships. Figure 7.19 displays the general global pattern of land potential evapotranspiration.

Fig. 7.19. Global annual mean potential evapotranspiration. Contour units in cm. (NCEP Reanalysis data courtesy of NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their website at http://www.cdc.noaa.gov/.)

Potential evapotranspiration varies relatively smoothly with a broad maximum in the tropics and decreasing with increasing latitude. Northern Hemisphere ETp is about 59% greater than the Southern Hemisphere total due to the greater land area in the Northern Hemisphere. The major exception to the general hemispheric pattern is that ETp for the latitudinal band from 0° to 20° S exceeds ETp for the band from 0° to 20° N by 7%. ETp is greater in the Northern Hemisphere for all other latitude bands.

The pattern within latitudinal bands displays complexity as factors other than solar radiation exert an influence on the maximum evapotranspiration quantity. The region in Africa between 10° N and 20° N is a good illustration of how the systematic influence of solar radiation is altered by seasonal variations in atmospheric circulation and cloud cover that influence available energy and humidity. Available energy is especially abundant in this zone due to the directness and duration of sunlight relative to latitudes further north or south.

ETp indicates the maximum moisture loss from the land surface, but the moisture flux that incorporates climate, soils, and vegetation influences is ETa. In most instances, ETa is less than ETp as discussed in Section 4.8.3, and it represents the actual moisture flux to the atmosphere from land areas. However, the ET process is limited by soil moisture and plant conditions for most land areas, causing the total moisture flux from the continents to be less than from the oceans (see Table 2.2). Oceans contribute 6-times the moisture input to the atmosphere by evaporation compared to the volume land areas provide through ETa. The oceans benefit from their surfaces providing an unlimited moisture supply and from the abundant energy over the topical oceans to drive the evaporation process.

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