Soil water storage in dryland cropping systems: The significance of cropping intensification

Soil and water conservation is essential to the sustainability of Great Plains dryland agriculture. We hypothesized that cropping intensification improves the efficient use of precipitation. We evaluated long-term observations of soil water at three locations in eastern Colorado for a range of pan evaporations (1050-1900 mm), soils, and cropping systems, Soils at various locations were mostly of the Argiustoll subgroup except for one Ustochrept and one Haplargid, both at the higher evaporation location, Normal precipitation at the three locations ranges from 400 to 425 mm yr(-1). Systems included a 2-yr winter wheat (Triticum aestivum L.)-fallow (WF) and more intense 3-yr winter wheat-corn (Zea mays L.)fallow and winter wheat-sorghum [Sorghum bicolor (L.) Moench]-fallow and 4-yr rotations. To quantify the effectiveness of the intensified systems at utilizing precipitation, we introduce the System-Precipitation-Storage Index (SPSI) and System-Precipitation-Use Index (SPUI), Mean SPSI values were 0.19 and 0.28 for 2- and 3-yr systems, respectively, meaning that the fallow periods in the 3-yr rotation were collectively 47% more efficient at storing precipitation than fallow in WF, Inclusion of a summer crop, such as corn or sorghum, increased the fraction of precipitation allocated to growing-season crop production (i.e., SPUI) from 0.43 in WF to 0.56 (i.e,, an increase of 30%) in 3-yr systems. The gains in efficient use of precipitation with intensification resulted from (i) reducing the frequency of the inefficient fallow preceding wheat, and (ii) using water for transpiration that,would otherwise be lost during fallow through soil evaporation, runoff, and deep percolation.