Response of ET of Irrigated Urban Landscapes to Coherent Changes in Summer Climate

Water consumed by irrigated plants in the western region of the US is by far the largest component of water use, and both irrigated agricultural and urban landscapes are the major players. This represents a critical issue in planning and managing water resources in an uncertain future. This is especially true in the intermountain region (IM), which is experiencing rapid population growth, coincident with projections of reduced available water. This region is generally characterized by warm/hot and often dry summer seasons with high radiation, resulting in substantial water use by irrigated plants. In such conditions, several key climate properties have a massive influence on the water use each year. Two very critical issues present themselves. First, the need for valid measurements and models of the actual ET of the irrigated ecosystems. Second, the change in the ET resulting from future user “demands” and climate. These are connected as user demand is highly determined by the climate.

Here we address the connections between coherent changes in the summer climate, and the resulting changes in future water use demand for irrigation. The objectives are:

1. Quantify significant temporal changes and patterns of summer climatic properties that govern ET
2. Combine measurements and models to estimate changes of ET in response to coherent patterns and changes in climate

Our analyses demonstrate harmonic and coherent temporal patterns in summer temperature and saturation deficit in the intermountain region. In the last 30 years, these cycles have become more amplified while temperatures are getting hotter. The simulations of climate models have consistently insisted that summers will continue to grow hotter in this region over the coming decades. The extreme nonlinearity of the Clausius Clapeyron Equation relating saturation vapor pressure to temperature, requires that even small increases in temperature will induce large increases in “atmospheric demand” for ET. However, the situation is made more complex, since plants respond to changes in saturation deficit by altering their stomatal conductance.

Estimating how ET will respond to near-term and long-term variations in climate, requires integration of synoptic climatology of summer conditions with a diagnostic model for ET. Currently, we operate a number of eddy covariance systems in the IM region to quantify actual ET and energy balance over irrigated surfaces. Here we focus on irrigated turfgrass, the main urban water use. These data can be used with the actual Penman-Monteith (PM) equation to determine the bulk canopy stomatal conductance. These results can also be used to test several models of stomatal conductance, and the sensitivity to changes in atmospheric conditions. Finally, climate simulations and PM equation can be combined, in a diagnostic way to simulate future changes in water demand by irrigation, accounting for both climate and biophysical processes.