Turning Stored Water into Crop Production
The effect of no-till on precipitation storage is shown in Figure 4.5. During the fallow period after wheat harvest (fall through the winter), little precipitation storage occurs in a typical stubble mulch system due to water use by weeds. In the spring and second summer of the fallow period, farmers till to control weeds and precipitation storage occurs, reaching a maximum in early September. In addition to evaporation losses due to the high summer temperatures, each tillage operation results in loss of stored soil water, with the amount dependent on the type of tillage (Table 4.4). About 25 percent of the precipitation received during the entire fallow period is stored under a typical stubble-mulch tillage system. However, 40 to 60 percent of precipitation can be stored using no-till due to reduced evaporation and weed management during the previous crop and the entire fallow period. Soil surface residues decrease evaporation rates by lowering soil temperatures, which increases precipitation storage. The soil water content reaches a maximum by June, and no additional precipitation storage occurs for the remainder of the summer fallow period because the soil has reached its maximum water-holding capacity under no-till management. However, as shown in Figure 4.3, most of the precipitation occurs after May. If the soil profile is full, most ofthe precipitation is lost to evaporation, deep percolation, or runoff.
The increased soil water stored under no-till usually does not result in a significant increase in wheat yields in a WF system under "normal" rainfall conditions because the soil profile in a stubble-mulch system will be full in the fall. However, under drought conditions, increased wheat yields may occur in a WF system under no-till. Under normal rainfall conditions, crop diversification is required to take advantage of the increase in water storage efficiency in no-till systems. Increased crop production and profit are achieved by integrating a summer crop into the production system.