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Turbid Waters Rise: How Napas Operate

S4 Climate Reports: Why use nappa water in dry years and reduce the load in wet years?

July 13, 2019

The increase in the water level in the soil was deepened more by land use than by rainfall. Producer challenges.

“In Argentina, the greater rainfall added to changes in soil management produced an increase in water tables, which translated into an increase in floods,” said Carlos Di Bella, an expert in agrometeorology.

Faced with this, what can the producer do? “Take advantage of the opportunity by using water from the nappa in dry years and reducing its recharge in wet years, as well as earlier plantings, double grain crops or services, and faster and earlier relays between them,” responded Jorge Mercau of the Inta Rural Station Agency in San Luis and member of the Environmental Studies Group (GEA-Imasl-UNSL).

Both researchers raised the issue in the July report prepared by S4, the Argentine company of technology applied to agriculture.

Di Bella pointed out that in many Argentine plains there are important changes in the inter-annual rainfall, and in addition, in many of these areas there were changes in the use and coverage of the soil, such as the area planted. “These changes, which interact simultaneously in much of these ecosystems, led to an increase in the level of groundwater tables and consequently an increase in the intensity and frequency of flooding” (García and others 2018).

The presence of groundwater near the surface provided water in situations of water deficit in summer crops during critical periods. Jobbagy et al. (2007) and Nosetto et al. (2009) presented evidence of the influence of nappa depth on crop productivity, defining maximums in winter crop yields when the depth is between 0.70 1.65 metres or in general terms between 1.2 and 2.5 m for summer crops.

These relationships depend on genotypes and soil type.

land use. Mercau indicated that in the last 50 years in great part of the plains pampeana and chaqueña the rains show an important interannual variability. However, although in some areas there were periods of several years more humid or drier, there is no clear trend in that period. On the other hand, there was a clear change in land use.

The growth of agricultural area to the detriment of areas occupied by pastures, grasslands and natural forests is the clearest and most forceful change in the hydrology of both plains. Unlike annual crops, perennial cover crops have a greater capacity to consume water through evapotranspiration 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.

On the other hand, within agriculture, the reduction of double crops for economic reasons, and the careful search for the reduction of the risk of drought, through the delay of sowing dates (notably in maize), further reduce the evapotranspiration capacity and increase the frequency of water surpluses that recharge the water table.

Faced with this, “the rapprochement of the layers in vast areas of the main productive systems, is one of the most important opportunities to increase production levels and, at the same time, one of the most important threats to heritage and infrastructure, when it is transformed into floods and salinization of the fields. Even in some areas with slightly steeper slopes, this rapprochement of the napa gives rise to phenomena of collapses that lead to the emergence of new rivers,” he said.

To avoid problems and take advantage of the opportunity, it is important to use water from the napa 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. In environments where variations in the water table level 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 water and proximity to the water table through an adaptive intensification of agriculture.

Mercau also advised earlier plantings, double grain crops or services, and faster and earlier relays between them, which allow consumption to increase between 100 and 200 mm over a single later crop and raise raises maintain a water table level fluctuating between 1.5 and 3.5 meters.

 

Source: La Capital