Reciprocal influence between crops and shallow ground water in sandy landscapes of the inland Pampas
Authors: Nosetto, MD, EG JobbÃ¡gy, RB Jackson, GA Sznaider
In regions with shallow water tables, ground water may have a positive (water supply) or negative (waterlogging or salinization agent) impact on crops. Reciprocally, crops can influence ground water, altering water table depth and chemical composition. We quantified these reciprocal influences along natural gradients of groundwater depth in flat sedimentary landscapes of the Inland Pampas occupied by wheat, soybean, and maize during two growing seasons (2006/2007 and 2007/2008). We correlated crop yield and groundwater depth maps at the field level and made direct plant, soil and groundwater observations at the stand level across topographic gradients. Water table level largely accounted for spatial crop yield variation, explaining 20-75% of their variance. An optimum groundwater depth range, where crop yields were highest, was observed for all three crop species analyzed (1.40-2.45 m for maize, 1.20-2.20 m for soybean, and 0.70-1.65 m for wheat). The areas within these optimum bands had yields that were 3.7, 3 and 1.8 times larger than those where the water table was below 4 m for wheat, maize, and soybean, respectively. As groundwater levels become shallower than these depth bands, crop yields declined sharply (~0.05 kg m-2 on average for every 10 cm increase in water table level), suggesting negative effects of waterlogging, root anoxia and/or salinity. Groundwater levels below these depth bands were associated with gradually declining yields, likely driven by poorer groundwater supply.
Crops influenced groundwater levels through their control of recharge and discharge fluxes. The presence of active crops prevented recharge events (sharp water table level raises) observed during rainy periods in fall and spring. Crops consumed groundwater generating an intensifying discharge as the water table depth decreased. This consumption lead to rising soil and groundwater salinization towards Q1 shallower water table positions as the growing season progressed. The electrical conductivity of ground water for maize at maturity doubled the pre-sowing values (~2.2 dS m-1 vs. ~1.1 dS m-1, p < 0.01) when groundwater was above 2-m depth; whereas negligible changes were observed when groundwater depth exceeded 3.5 m. In flat humid landscapes, as the Inland Pampas, crops and shallow ground water may be closely connected and influenced each other through different mechanisms, posing both opportunities and risks for agricultural systems. Understanding these complex interactions could help raise and stabilize yields and provide keys to regulate the labile hydrology of these plains