158 THE FUTURE OF ARID LANDS 
not encourage natural water spreading) and vegetation (which 
can modify infiltration rates), were also found to be influential. 
However, no single watershed characteristic that consistently 
correlated with high or low water yields could be found. This may 
be due in part to the fact that each watershed had a different com- 
bination of soils, vegetation, physiography, and other factors, any 
of which might play a greater or lesser part in one watershed than 
in another. 
Soon after the study was started, however, it became apparent 
that precipitation, in combination with evapotranspiration poten- 
tials, consistently correlated with watershed yields throughout a 
wide range of watershed conditions. True, it was found that, all 
other things being equal, such watershed conditions as have been 
mentioned heretofore could materially influence the yield of water; 
nevertheless, the apparent influence of the precipitation-evapo- 
transpiration factor far overshadowed watershed influences. For 
example, it was found that with low precipitation and high 
evapotranspiration perhaps 1% of the annual precipitation might 
be expected to appear as water yield. On the other hand, and con- 
sidering all zones covered by the study, with high precipitation and 
low evapotranspiration, perhaps 50% of the precipitation might 
appear as water yield. It became obvious, therefore, that these 
two factors were predominant in the yield of water to the extent 
that watershed characteristics, as such, ‘‘fell out of the picture.” 
As mentioned by Thornthwaite (1) there has been published a 
generalized map of the evapotranspiration potentials of the four 
states considered in this study. Although the map is not of such a 
scale as to permit a high degree of accuracy, it was possible to 
develop approximate evapotranspiration potentials from the map 
for comparative purposes. 
Considering only the arid zone, the weighted evapotranspira- 
tion potential was found to be about 29 inches on the average, 
annually. This can be compared with the weighted precipitation 
of 13 inches. To phrase this another way: on the average, there 
exists a potential for evaporation or transpiration of some 220% 
of the average annual precipitation. Although this comparison 
disregards seasonal fluctuations in evapotranspiration or precipi- 
