ABSTRACT
Several studies improved upon this analysis and compared the yields of a given crop under four different scenarios: traditional practices, water harvesting only, fertilizer use only, and a combination of water harvesting and fertilizer use. These studies examined maize (Barron and Okwach, 2005) and sorghum (Fox and Röckstrom, 2000, 2003; Carsky et al., 1995; Carter and Miller, 1991) production under the above scenarios. The general finding was that water harvesting alone can boost yields, but the greatest increase in yields occurs when water harvesting is combined with fertilizer use. As with Tabor (1995), these increases depend on the total rainfall realized during the growing season. The exception to these results is Carsky et al. (1995), who find that water harvesting alone boosts yields more than when combined with fertilizer use. Finally, Zougmoré et al. (2004) compared sorghum yields from two separate water harvesting techniques under three scenarios: water harvesting without fertilizer use, fertilizer use without water harvesting, and the combination of water harvesting and fertilizer use. They concluded that the combination of water harvesting and fertilizer use increases water-use efficiency and increases yields. The above studies were primarily field experiments that examined the relation between inputs and an output, with or without water harvesting. Another set of studies used more sophisticated models designed to capture the economic benefits of water harvesting. One common technique is gross margins analysis, which is conducted using the following deterministic relationship:
GM n
P V VCjk ij i
ijk ijk= −
∑1jk ij
n
P V Vjk ij i
ijk ijk= −
1 GM
n P V VCjk ij
ijk ijk= −
∑1 .
