GROUNDWATER UPTAKE AND SUSTAINABILITY OF ACACIA AND PROSOPIS PLANTATIONS IN SOUTHERN PAKISTAN

Farm woodlots or plantations of salt tolerant trees may provide an eco nomic use or reclamation treatment for salt-affected farmland within t he irrigation regions of the Indus Valley, but the hydrological impact and sustainability of such plantations are unknown. Detailed measurem ents of plantation water use, watertable depth and soil conditions wer e recorded over 2 years in two small plantations with contrasting soil and groundwater salinity at Tando Jam in the Sindh province of Pakist an. The species monitored were Acacia nilotica, A. ampliceps and Proso pis pallida. Annual water use by 3- to 5-year old A. nilotica was 1248 mm on the severely saline site and 2225 mm on the mildly saline site. Water use by the other species was less than 25% of these rates, but this difference is largely explained by their lower density in terms o f sapwood area per hectare. Water use by A. nilotica was considerably greater than annual rainfall, implying uptake of groundwater which was confirmed both by piezometric observations and chloride balance model ling to predict vertical water movement through the root zone. Plantat ion watertables fell from 1.7 m below surface in March to over 2.9 m i n September, then rose again during irrigation of the surrounding farm land. Root zone salt concentrations remained high at the more saline s ite throughout the monitoring period, but at the less saline site ther e was evidence of increasing root zone salinity as salt accumulated in areas of the profile subject to root water uptake. Salt concentration in the upper profile decreased as the soil dried and water was absorb ed from greater depth. Plantations using saline groundwater may be sus tainable if occasional leaching and other salt-removing processes are sufficient to maintain root zone salinity at a level which does not ex cessively reduce tree growth. (C) 1998 Elsevier Science B.V.