The phreatic rapprochement of vast areas of the country’s productive systems is a strength, but also a weakness when patrimonies are threatened with floods and salinization of fields.
“When we see the water table close to the surface, it is necessary to assemble agroecosystems with high productivity and with characteristics that allow us to take advantage of their opportunities and avoid the problems they can bring. The design of these adaptive strategies is, without a doubt, one of the most important tasks in the productive assembly of a company.
The agronomist M.Sc. Jorge Mercau, of the AER of INTA San Luis and member of the Environmental Studies Group (GEA-IMASL-UNSL), said it about the rapprochement of napas in vast areas of the main productive systems of the country.
“It is one of the most important opportunities to increase production levels but, at the same time, it is one of the most important threats to heritage and infrastructure, when it is transformed into floods and salinization of the fields,” he added.
“Even in some areas with slightly steeper slopes, this rapprochement of the napa gives rise to phenomena of collapses that lead to the appearance of new rivers,” said Mercau in a report by S4, the Argentine company of technology applied to agriculture.
Mercau has insisted that, in order to avoid problems and take advantage of the opportunity, it is important to use water from the nappa in dry years and reduce its recharge in wet years.
“In both cases, using water by increasing transpiration leads to proportional increases in grain and/or biomass production,” he said.
“In environments where variations in the level of the water table are low, due to a high proportion of large pores in the soil, and where it does not reach high salinity values, it is feasible to take advantage of the water and proximity of the table through an adaptive intensification of agriculture,” he explained.
“Earlier plantings, double grain crops or services, and faster and earlier relays between them, allow consumption to increase between 100 and 200 millimeters over a single later crop, and, with adaptive decisions that manage the risks of droughts and excesses, put forward proposals that maintain a water table level fluctuating between 1.5 and 3.5 meters,” he said.
Mercau also said that in the wetter areas it will be necessary to have a more aggressive strategy that adaptively involves, in time and space, perennial pastures, especially in the lower parts of the landscape.
“The situation is more complex in environments where salinization is more likely, especially if variations in water table levels are high, due to a low proportion of large pores in the soil,” he said.
“That is the situation we are detecting in the Chaco region, where the change in use is more contrasting, and where the challenge of maintaining that layer far enough requires greater engineering of the production system and, along with other arguments, reinforces the need to drastically reduce the replacement of forests by agricultural systems with annual crops,” he said.
One of the biggest debates is whether the increase in water tables is only due to greater rainfall or whether it is influenced by changes in land use and cover.
“In the last 50 years, in much of the Pampean and Chaco plains, rainfall has shown significant inter-annual variability. However, although in some areas there were periods of several years that were wetter or drier, there is no clear trend in that period,” said Mercau.
“Instead, there was a clear change in land use. The growth of the agricultural area, to the detriment of areas occupied by pastures, pastures and natural forests, is the clearest and most forceful change in the hydrology of both plains,” he said.
“Unlike annual crops, perennial cover crops have a greater capacity to consume water through evapotranspiration – going from 600-800 millimeters to 900-1,200 mm – and avoid excesses, but they also have deeper roots, especially trees, which allow them to use in dry years the excess of previous wet years, going from an exploration of 2 meters to one of 4 to 10 meters or more,” he said.
Mercau also said that, within agriculture, the reduction of double crops for economic reasons, and the careful search to reduce the risk of drought, through the delay of sowing dates, especially in maize, further reduce the evapotranspiration capacity of the productive systems, and increase the frequency of water surpluses that recharge the water table.
“In the plains, that recharge has led to an inexorable approach to the surface,” he added.
Soil and cover
According to agricultural engineer Carlos Di Bella, a specialist in agrometeorology, in many Argentine plains important modifications are registered in interannual rains.
“In many of these areas there were changes in the use and coverage of the soil, as well as the area planted,” he said.
“These changes, which interact simultaneously in a large part of these ecosystems, produced an increase in the level of the water table and, as a consequence, an increase in the intensity and frequency of the floods. In those cases, the effect of the rains was greater than the change in land use,” he added.
“At the same time, the presence of groundwater near the surface provided water in situations of water deficit in summer crops during critical periods,” he said.
Di Bella also said that – in publications by Jobbagy and others (2007) and Nosetto and others (2009) – evidence was presented about the influence of the depth of the nappa on crop productivity.