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To understand the effect of restoration thinning on the water balance of upland semi-arid ponderosa pine (Pinus ponderosa) forests of the southwestern US, we compared the components of forest water balance between an unthinned plot and a thinned plot using a paired water balance approach. Forest overstory transpiration (E-o) was estimated from tree sapflow scaled to the plot level. Understory evapotranspiration (E-u) was estimated from the difference between throughfall precipitation and changes in soil water content measured in trenched plots that excluded tree roots. The thinning treatment in 2001 reduced plot basal area by 82% and leaf area index by 45%. Difference in stand-level evapotranspiration (E) between the thinned and unthinned plots, and partitioning of E between E-u and E-o during the first post-treatment summer and spring, varied between drought and non-drought periods. The importance of E-u in stand-level E was greater in thinned compared with unthinned plots and increased during extreme drought when E-o was low due to stomatal closure. Our results highlight the importance of drought and climate as factors determining the impact of thinning on water balance in southwestern ponderosa pine forests. (c) 2007 Elsevier B.V. All rights reserved.