Groundwater, threats and opportunities: experiences of producers in the Pampas region

María Eugenia Magnelli, Agricultural Engineer. (Aapresid Press) 

In recent years, the Pampas region has been affected by water surpluses and rising phreatic  water tables, which has become an opportunity that, if not taken, it becomes a threat. 

Within the framework of the 29th Aapresid Congress "Siempre Vivo. Siempre Diverso." (Always Alive. Always Diverse.), the agricultural engineer Suyai Almirón (Technical  Coordinator of Sistema Chacras Aapresid) let the members of Chacras Justiniano Posse,  América and Los Surgentes share their experiences. 

From the south of the province of Córdoba, Fernando Tous (Chacra América) explained  that in 2008, they began to evaluate the groundwater depth to know its variability and, for  this purpose, they installed six phreatic water level meters in different plots of the field. The  soils in the region are flat with little slope and an average rainfall of 800 mm, with a dry  season in winter. 

Contrasting the precipitation values with the data provided by the phreatic water level  meters for several years, they saw an upward and downward movement of the water table.  In rainy years, it reached up to 40 cm and in drier years, it dropped to 2 meters. 

Based on this, they designed a digital model of different basins within the project of  Chacras, in order to be able to forecast the surface movement of water surpluses in the lot,  evaluate the management that is carried out in each situation and plan the rotation in  advance. 

The conclusion was that the intensification of the rotation allows them to avoid further soil  deterioration and to start the period of precipitation concentration (spring-summer) with a  more distant water table, which helps water surplus to infiltrate the soil. 

In the case of Chacra Justiniano Posse, Francisco Rubio said that in the last 100 years,  precipitation increased from 750 mm to 900 mm. This scenario and the water table near the  surface, transformed many unproductive areas, which was also generated by the change in  land use. 

Faced with these challenges, at the regional level, they began to channel runoff channels to  evacuate quickly water surplus.

They also intensified rotations to keep the water table at bay and prevent the capillary rise  of saline water. In this sense, they learned that they should think of it as something flexible,  planning it year by year according to the needs of the lot and the profitability of the grains.  Under this scheme, summer crops have been more stable in their yields. 

Comparing vicia and wheat, Rubio commented that the legume as a service crop has the  advantage of contributing biological nitrogen and biodiversity to the system. The downside  is that it affects soybean yields in the short term, in addition to increasing rotation and  logistics costs. In the case of cereals, they obtain a higher gross margin per hectare, good  weed control and greater stability in second soybeans. The disadvantages are the few cold  resistant materials in pasture, great variability in wheat yields and greater dependence on  nitrogen fertilizers. 

In the southeast of the province of Córdoba, Héctor Miotti (Chara and Regional  Aapresid - Los Surgentes Inriville) said that the topography of the land has little slope,  and in times of heavy rainfall, with the water table nearby, the water has no outlets in the  lot, "they are all puddles," he said. 

By analyzing the phreatic water level meters and rainfall, they found that the water table  moved differently in the same situations. And it is there, when in 2010 they decided to form  the Chacra. Together with INTA Manfredi, they began to analyze aerial images to see in  which areas water has more infiltration and in which less. 

The digital models allowed them to see the water surpluses and with this tool, they adjust  the species that form rotation. In some environments, they plant perennial pastures, tall  wheatgrass, fescue and ryegrass in the worst sod patches. In others, they practice double  cropping, they plant vicia in places that are not waterlogged and sorghum with very good  results. Thus, they were able to consume the water, and with more ground cover, they limit  evaporation. 

A common issue in all three cases is the outcrop salinity at the surface. Soil and water table  analyses (salinity, exchangeable sodium ratio and pH) have allowed them to identify the  situation of each environment and adjust management practices. 

Reinforcing previous experiences, Horacio Vilella (INTA Laboulaye) said that, knowing  the characteristics of the type of soil and water table, it is possible to define environments  ranging from high to low productivity and adopting different strategies accordingly. 

Vilella spoke of optimal (EC less than 4 dS/m, low sodium and depth greater than 1 m) and  lethal (EC greater than 4 dS/m, high sodium and depth less than 1 m) water tables that  interacts with plant roots. 

The strategy to reduce the capillary rise of saline water and water is to have crops all year  round. In order to increase infiltration, it is essential to generate cover and eliminate  physical obstacles to soil of anthropic origin.

As a final consideration, Vidella emphasized, "the generalized rise in groundwater levels in  a very broad region has generated changes in agroecosystems that require an understanding  of the groundwater-soil-crop-climate functioning specific to each place. Adapting crop  management to the groundwater recharge situation of each environment is key to be  economically profitable and environmentally sustainable."