Effects of differential irrigation on water use efficiency, canopy photosynthesis, and aspects of flux relations of maize fields

Experiments were conducted to study effects of water stress on water-use efficiency (WUE) of maize in Davis, California during three consecutive years (1994 to 1996). Adjacent fields were given different irrigation treatments, one was well irrigated (WET) and the other received limited irrigation to produce stress (DRY), which was different each year.; The soil water content, soil CO₂ efflux, biomass accumulation, leaf area index, light interception, leaf water potential, and leaf nitrogen were monitored through the season. Canopy net photosynthesis (A _c) and evapotranspiration (ET) were measured with a Bowen ratio-energy balance system enhanced to measure the CO₂ flux (BREB+). Measurements of canopy temperature (T_c) were used to calculate canopy conductance (g_c). Early in the 1996 season, soil surface temperature and humidity were measured to assess energy balance of the soil.; Low moisture content in the upper soil layers in DRY caused insufficient N uptake, which resulted in a lower leaf N, and reduced A_c. Complications induced by dry soil on the crop nutritional status are discussed.; Seasonal water use estimated with the BREB+ was less than estimated with the soil water balance in WET but not in DRY. The difference is thought to be due to deep percolation. WUE determined by biomass sampling and soil water balance (WUE_me) appeared to be reduced by water stress only if water loss by deep percolation was estimated.; Canopy size had an overriding effect on ET, g_c, and A_c. Reductions of ET and g_c in DRY could be explained almost totally by reduced leaf area. Diurnal trends in A_c and ET for many days in 1996 are presented. Water stress reduced A_c more thanET, and hence reduced photosynthetic WUE (WUE_ae = A_c/ ET). WUE_ae was reduced by water more than was WUE _me. Reasons for this difference are explored.; Additional topics treated are: generation of buoyancy by bare soil and energy exchange between the soil and the crop; testing of Penman-Monteith equation by comparing calculated T_c and g_c with measured values of T_c and g_c; and behavior of Bowen ratio in relation to the extent of canopy cover.