Assessing Future Climate and Land Use Impacts to the Colorado River using a Bottom-up Modelling Approach Informed by Water Managers

Accelerated climate and land use land cover (LULC) changes are anticipated to have large impacts on water resources in the Colorado River Basin (CRB). Since land use is a result of complex socio-ecological factors, accurately predicting future patterns of LULC is challenging. In addition, substantial differences among a large number of climate models necessitate a screening process for impact and adaptation studies. As a result, limitations of conventional ‘top-down’ approaches are becoming increasingly apparent. More recently, ‘bottom-up’ assessments are gaining popularity for exploring climate and LULC conditions using a few selected cases that consider a range of possibilities. Here, we improved and employed the Variable Infiltration Capacity (VIC) model to generate streamflow projections across the CRB under multiple cases of climate and LULC changes. We integrated advances in the model using Landsat- and MODIS- based products to produce more realistic land surface conditions. Meteorological datasets were drawn from statistically downscaled projections (2006-2099) to represent ‘hot and dry’ and ‘warm and wet’ futures. Vegetation parameters were modified by using regional projections of a LULC model upon which more drastic disturbances were applied to forest cover types. Water managers in the CRB were consulted to ensure that a range of views were captured in the modeled storylines and to maximize the legitimacy and credibility of the research for decision-makers. Analyses were conducted to identify system vulnerabilities and unexpected outcomes that pose the greatest consequences to long-term water supplies in the CRB. Results indicated that forest disturbances partially offset warming effects to streamflow (basin-wide mean annual streamflow was up to 9% larger than the case without disturbance by end of century), allowing more neutral impacts under warming.