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Universidad Nacional de La Plata (UNLP) 2014

Evaluación de la influencia hidrológica de forestaciones en la llanura pampeana

Besteiro, Sebastián

Titre : Evaluación de la influencia hidrológica de forestaciones en la llanura pampeana

Auteur : Besteiro, Sebastián

Université de soutenance : Universidad Nacional de La Plata (UNLP)

Grade : Doctor de la Facultad de Ciencias Agrarias y Forestales 2014

Contexte
Since the XIXth Century the importance of including afforestation in the Argentinean Pampas has been highlighted because of its unique capacity to modify the water dynamics of these environments. The low slopes, the saline-sodic soils and the shallow water table constitute a very fragile environment threatened by the progressive advance of agriculture and intense animal use. All these characteristics determine long periods of drought followed by floods with severe consequences to the livestock production. Under these conditions erosive processes depend largely on changes in land use and productive activities. Worldwide demand for forest products as well as fiscal support has turned the attention towards these environments for the development of afforestation. Nevertheless, the effect of afforestation is not clearly evident due to the fact that it has been introduced in farms in a circumscribed and isolated way. A proper understanding of the effect of afforestation in the water system can help to find ways to improve the management of water, and hence, to improve the productive activities that are very often affected by floods and droughts. The aim of the present study is to evaluate the hydrological effects of fast growing afforestation in two localities of the pampean prairie in order to identify criteria for the management of water in plain environments. During a three year period the different components of the water balance were determined and analyzed in afforestation of Pinus radiata Don and Eucalyptus viminalis Labill in the rolling pampa and in Eucalyptus camaldulensis Dehnh and Eucalyptus viminalis Labill in the flooding pampa. With this purpose the following specific objectives were established : 1) To analyze the rain distribution under adult afforestation of Pinus radiata Don, Eucalyptus viminalis Labill and Eucalyptus camaldulensis Dehnh in two localities of the pampean prairie ; 2) To model the runoff under current land use conditions and simulate possible scenarios considering a change in land use (conversion from pasture to afforestation) ; 3) To evaluate fluctuations in the water table at farm level to determine relationships of charge/discharge under the land uses considered in each studied site, and 4) To establish a simple methodology to estimate the crop coefficient in order to quantify the evapotranspiration of the considered afforestation. The results obtained in relation to rainfall indicate that pines have a higher interception capacity than eucalyptus, which is directly related to the age of the stands. Using the methodology of the curve number by means of the hydrological model L-THIA NPS, an important reduction of the annual runoff in the conversion of pasture to afforestation was established. This model proved to be a powerful tool for the diagnosis of superficial water runoff behavior in hypothetical situations of land use. At an underground level, it was confirmed that the afforestation studied in the flooding pampa increased infiltration processes in periods with excess water, and also increased water deficit in periods with low rainfall because of increased phreatic water discharge. In contrast the afforestation of the rolling pampa decreased water discharge during dry seasons or low rainfall and increased infiltration processes in periods of excess water. It was observed that while some stands acted as preferential sites for discharge, others were able to facilitate or interrupt recharge when compared with other land uses or coverage. Finally, the methodology developed to calculate the evapotranspiration demonstrated that the conversion from pasture to fast growing afforestation generated an increase of the evapotranspiration of 51-87% when compared to the natural situation and turned the hydric performance from positive to strongly negative for most of the time. This represented an extra contribution of 35-67% above the median annual precipitation, establishing that the pine stand has a minor water yield (groundwater discharge) than eucalyptus, and within the latter, that the stands of E. viminalis have a higher water yield than E. camaldulensis. Finally it is concluded that afforestation make a more exhaustive use of water when compared with pastures. The canopy intercepts more water and reduces the amount that reaches the soil, the water that reaches the soil does not drain superficially so easily because of the litter and tends to infiltrate in the soil. This infiltration, facilitated by an increase of the porosity and hydrophobicity of organic matter, is quickly absorbed by afforestation and released to the environment. The existence of a phreatic aquifer at less than 6m from the surface constitutes an extra supply of water for afforestation that is used when needed, generating depressions of more than 2m deep and turning natural sites of recharge in sites of net discharge. One of the main problems related to water consumption by plants is the possibility of the salinization of soil and water tables. This issue has been treated in local studies and there is a clear need to keep on studying this matter.

Mots clés : forestaciones de rápido crecimiento ; balance hidrológico

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Page publiée le 31 août 2016, mise à jour le 3 juillet 2017