Water subsidies from mountains to deserts: Their role in sustaining groundwater-fed oases in a sandy landscape

JOBBAGY EG; NOSETTO MD; VILLAGRA, PE; JACKSON RB

ECOLOGICAL APPLICATIONS

ECOLOGICAL SOC AMER

Año: 2011 vol. 21 p. 678 - 694

1051-0761

In arid regions throughout the world, shallow phreatic aquifers feed naturaloases of much higher productivity than would be expected solely from local rainfall. In SouthAmerica, the presence of well-developed Prosopis flexuosa woodlands in the Monte desertregion east of the Andes has puzzled scientists for decades. Today these woodlands providecrucial subsistence to local populations, including descendants of the indigenous Huarpes. Weexplore the vulnerability and importance of phreatic groundwater for the productivity of theregion, comparing the contributions of local rainfall to that of remote mountain recharge thatis increasingly being diverted for irrigated agriculture before it reaches the desert. Wecombined deep soil coring, plant measurements, direct water-table observations, and stable isotopic analyses (2H and 18O) of meteoric, surface, and ground waters at three study sitesacross the region, comparing woodland stands, bare dunes, and surrounding shrublands. Theisotopic composition of phreatic groundwaters (d2H; 137% 6 5%) closely matched thesignature of water brought to the region by the Mendoza River (137% 6 6%), suggestingthat mountain-river infiltration rather than in situ rainfall deep drainage (39% 6 19%) wasthe dominant mechanism of recharge. Similarly, chloride mass balances determined from deepsoil profiles (.6 m) suggested very low recharge rates. Vegetation in woodland ecosystems,where significant groundwater discharge losses, likely .100 mm/yr occurred, relied onregionally derived groundwater located from 6.5 to 9.5 m underground. At these locations,daily water-table fluctuations of ;10 mm, and stable-isotopic measurements of plant water,indicated groundwater uptake rates of 200–300 mm/yr. Regional scaling suggests thatgroundwater evapotranspiration reaches 18–42 mm/yr across the landscape, accounting for 7–17% of the Mendoza River flow regionally. Our study highlights the reliance of ecosystemproductivity in natural oases on Andean snowmelt, which is increasingly being diverted to oneof the largest irrigated regions of the continent. Understanding the ecohydrological couplingof mountain and desert ecosystems here and elsewhere should help managers balanceproduction agriculture and conservation of unique woodland ecosystems and the ruralcommunities that rely on them.