Background
The authors write that "quantifying the hydrological response to an increased atmospheric CO2 concentration and climate change is critical for the proper management of water resources within agricultural systems," which is precisely what they proceeded to do for California's highly agricultural San Joaquin watershed.

What was done
Ficklin et al. "modeled the hydrological responses to variations of atmospheric CO2 (550 and 970 ppm), temperature (+1.1 and +6.4°C), and precipitation (0%, ± 10% and ± 20%) based on Intergovernmental Panel on Climate Change projections," using the Soil and Water Assessment Tool (SWAT), which they describe as "a hydrologic/water quality model developed by the United States Department of Agriculture-Agricultural Research Service" for predicting "the impact of agricultural or land management on water, sediment and agricultural chemical yields in ungauged basins."

What was learned
In what climate alarmists would likely consider a worst case scenario -- an atmospheric CO2 concentration of 970 ppm and a 6.4°C increase in temperature -- the four researchers from the University of California at Davis report that these extreme values of the "twin evils" of the radical environmentalist movement caused watershed evapotranspiration, averaged over 50 simulated years, "to decrease by 37.5%, resulting in increases of water yield by 36.5% and stream flow by 23.5% compared to the present-day climate."

What it means
According to the results of Ficklin et al.'s study, not only would large increases in atmospheric CO2 concentration and air temperature not be bad for the hydrology of California's San Joaquin watershed, they would likely be good for it, which is just the opposite of what the world's climate alarmists typically contend.