Populations in upland catchments of northern Lao PDR are dependent on two distinct rice agroecosystems for food security: upland rice, i.e. traditional rainfed rice cultivation on steep slopes within a shifting cultivation system; and paddy rice, which is grown in valley bottoms and irrigated from available stream water. Intensive levels of shifting cultivation in recent decades have led to degradation of hillslopes and unsustainable upland rice production, threatening local rice sufficiency. Water flows link the two agroecosystems, but complex hydrologic effects from land use changes on slopes via alteration of the shifting cultivation system, and resulting water availability for irrigation in the valley bottoms, complicate intervention strategy formulation. This study presents results from a hydrologic modeling analysis of a 3.5 km2 study catchment in northern Lao PDR. The analysis deployed a physically-based, spatially-distributed hydrologic model of the land phase of the hydrologic cycle, including evapotranspiration, detailed surface water processes and simplified sub-surface flow representations. The model was used to assess changes to irrigation water availability for paddy rice from several land use change options that lessen shifting cultivation intensity and enhance forest cover. The study identified both potential trade-offs between upland rice and paddy rice production, and also some potential synergies in the amount and timing of available irrigation water that could enhance the productivity of paddy rice systems. Study results provide some initial indications of the effects of these changes on total rice production, as well as the associated costs and benefits to upland farmers. Implications of these results on poverty and food security are considered.