Including climate feedbacks in regional water resource assessments

Abstract

Two important changes to the meteorological conditions can occur as a result of a change in the landsurface state: the precipitation can change (Koster et al, 2004, Wang et al, 2007) and the evaporative demand can change (Ek and Holtslag, 2004, Schubert et al, 2004). Precipitation is particularly difficult to predict as it can be affected by large-scale weather patterns or complex processes such as mesoscale circulations or convective processes. The physics involved and the scale and complexity of the processes means that complex numerical atmospheric models are usually necessary to quantify impact of the land surface on precipitation, although a simple, analytical model can sometimes be used to assess the effect of the land- surface state on the likelihood of triggering convective precipitation (e.g., De Ridder, 1997). Changes in the evaporative demand are equally important. For instance, Cai et al (2009) have demonstrated the role that the land-atmosphere feedbacks have had on the recent Australian drought. The feedback link between the land surface and the evaporative demand is through the Planetary Boundary Layer (PBL) and this feedback process lends itself to a fairly simple analysis. A framework to assess the impact of change in the land surface on the evaporative demand, and subsequent available regional water resources is proposed in this report. This report addresses the following questions: 1. How do the feedbacks from the planetary boundary layer (PBL) control the surface water balance? What would happen if there were no feedbacks? 2. How much does land-cover to atmosphere feedbacks control the surface water balance? 3. How does irrigation change the atmospheric conditions? 4. How much does atmospheric CO2 affect the water-use? The tools and data we use to address these questions are selected Earth Observation fields, river and evaporation flux data to validate the land surface models, vertical profiles of temperature and humidity that control the evolution of the PBL, and various models: from 1-dimensional slab models of the PBL, 2-dimensional models of the land surface and 3-dimensional models of the surface and atmosphere. The four questions are addressed by quantifying the land-atmosphere feedbacks and the impacts of those feedbacks on the land-surface water balance using a unified framework