Effects of afforestation on groundwater recharge and water budgets in the western region of Uruguay

Abstract

Uruguay has stimulated the development of its forest sector since the promulgation of Forest Law N° 15 939 in December of 1987. Nevertheless, the substitution of natural grasslands with forest plantations for industrial use has raised concerns regarding hydrological processes of groundwater recharge and water consumption involving evapotranspiration. The purpose of this study is to assess the effects of this substitution approach on water resources. Input data were collected from two small experimental watersheds of roughly 100–200 hectares located in western Uruguay. The watersheds are characterized by Eucalyptus Globulus ssp. Maidenni and natural grasslands for cattle use. Total rainfall, stream discharge, rainfall redistribution, soil water content and groundwater level data were collected. Groundwater recharge was estimated from water table fluctuations and from groundwater contributions to base flows. Seasonal and annual water budgets were computed from October of 2006 to September of 2014 to evaluate changes in the hydrological processes. The data show a decrease in annual specific discharge of roughly 17% for mean hydrological years and no conclusive effects on annual groundwater recharge in the forested watershed relative to the reference pasture watershed. Reduced annual specific discharge is equivalent to the mean annual interception. The computed actual annual evapotranspiration is consistent with international catchment measurements. Reduction rates vary seasonally and according to accumulated rainfall and its temporary distribution. The degree of specific discharge decline is particularly high for drier autumns and winters (32 to 28%) when the corresponding rainfall varies from 275 to 400 mm. These results are of relevance for water resources management efforts, as water uses downstream can be affected. These findings, based on a study period dominated by anomalous wet springs and summers and by dry autumns and winters, oppose earlier results based on 34 years of rainfall and discharge data drawn from Uruguayan large basins. Copyright © 2016 John Wiley & Sons, Ltd.

Preface for the South American Hydrology Virtual Special Issue

“…This suggests that eucalypt plantations in these regions influence discharge regimes when compared with pasture land use and that partial harvesting could moderate the magnitude of discharge variation compared with a full catchment harvesting. Similar trends are reported by Silveira et al () studying the effects of afforestation on groundwater recharge and water budgets in the western region of Uruguay. The establishment of Eucalyptus plantations generated a decrease in annual specific discharge especially in dryer years.…”

“…Eight of the manuscripts tackle the effects of afforestation of grasslands and native forest substitutions and forest management in catchments covered with fast growing forest plantations; two of them approach rainfall partitioning analyses (Silva et al, ; Soto‐Schönherr & Iroumé, ), one analysed throughfall enrichment and stream nutrient chemistry in small headwater catchments (Hervé‐Fernández, Oyarzún, Brumbt, et al, ), two studied the hydrological effects of forest plantation management (Almeida, Smethurst, Siggins, Cavalcante, & Borges, ; Silveira et al, ) and other types of human interventions (Ochoa‐Tocachi et al, ), and one investigated water sources for native and exotic tree species (Hervé‐Fernandez, Oyarzún, & Woelfl, ). Silva et al () studied throughfall patterns in sugarcane and riparian forest to understand the effect of sugarcane age and land use conversion, finding no significant differences between cycles of sugarcane, growth phases, and riparian forest. However, as sugarcane presents high temporal variation of leaf area index, plant height, and leaf and culms structure or architecture, throughfall patterns of this crop should be analysed by growth phases to really clarify the effect of sugarcane on processes that influence the environment and agricultural yields.…”

“…South America has also a number of special features that make hydrology research particularly relevant, including moisture cycling in the Amazonian rainforest, the effects of the cold Humboldt current on precipitation, the steep elevation and orographic effects of the Andes, the mid-latitude temperate forests that resemble some northern hemisphere forests, a much smaller continental land mass and greater ocean effects on hydrology compared to the northern hemisphere, and a range of ecosystems from almost pristine to very Eight of the manuscripts tackle the effects of afforestation of grasslands and native forest substitutions and forest management in catchments covered with fast growing forest plantations; two of them approach rainfall partitioning analyses (Silva et al, 2016;Soto-Schönherr & Iroumé, 2016), one analysed throughfall enrichment and stream nutrient chemistry in small headwater catchments (Hervé-Fernández, Oyarzún, Brumbt, et al, 2016), two studied the hydrological effects of forest plantation management (Almeida, Smethurst, Siggins, Cavalcante, & Borges, 2016;Silveira et al, 2016) and found that all anthropogenic interventions increased streamflow variability and generated significant reductions in catchment regulation capacity and water yield. Hervé-Fernandez, Oyarzún and Woelfl (2016) assessed the "two water worlds" theory through a whole year in two catchments, one covered by Eucalyptus nitens and the other by a native evergreen forest, finding that the two water worlds does not apply throughout the year confirming previous hypotheses.…”

Forests and water in South America

“…Evapotranspiration in plantations varies with climate, season, forest type, and affected portion of the hydrograph (floods, low flows, groundwater; Almeida & Soares, ; Calder, ; Hervé‐Fernández et al, ; Lima, ; Lima et al, ). Evapotranspiration may exceed 90% of precipitation in plantations of Eucalpytus and Pinus spp., reducing water yield and depleting groundwater (Almeida, Smethurst, Sigins, Cavalcante, & Borges, ; Huber, Iroumé, & Bathurst, ; Jobbágy and Jackson 2004; Jobbágy et al, 2013; Silveira, Gamazo, Alonso, & Martinez, ; van Dijk & Keenan, ). Plantation forestry may not be cost‐effective when the value of evapotranspired water is taken into account (Chisholm, ; Núñez, Nahuelhual, & Oyarzún, ).…”

The effects of topography and forest management on water storage in catchments in south‐central Chile

“…A recent commentary has suggested that a key gap in the ability to generalize findings from paired watershed studies is the lack of understanding of subsurface catchment storage and how factors such as hydroclimate, topography, geology, and soil type conspire with catchment storage to define the watershed response to forest treatment (McDonnell, 2017). We know that in many cases, the establishment of forest plantations on former agricultural land reduces flow rates, especially during the dry season (Iroumé & Palacios, 2013, Moore & Wondzell, 2005, reducing not only water yield but also water storage availability (Andréassian, Perrin, & Michel, 2004;Silveira, Gamazo, Alonso, & Martinez, 2016). A decrease in the annual water yield, summer runoff, and peak flows is associated with higher transpiration rates caused by an increased proportion of the catchment being covered by forest and with the establishment, growth, species composition, or age of the forest (Andréassian, 2004;Best et al, 2003;Bosch & Hewlett, 1982;Brown, Zhang, McMahon, Western, & Vertessy, 2005).…”