Coherent Cycles of Summer Temperatures in the Intermountain West, and Effects on Urban Water Use

It has become clear that water resources are already becoming inadequate to address the demands in some rapidly growing regions of the western US. This is emerging as a large issue in parts of the Intermountain West. This especially includes Utah, with the highest per capita water use, and a rapidly growing population projecting to double in the next 30 years. There are two parts to the water resource equation in these cases. The first is the water provided by precipitation (mainly snowpack in many regions), which is not addressed here. However, the second, which is just as important, is the amount of water actually used, most of which is by plants during the summer season. The maximum summer temperatures play a critical role in the water “demand” for irrigation of urban landscapes and agriculture.

The typical approach to assessing any climate variables is to look at the 30 year averages. Analyses of the available instrument record reveals that summer maximum temperatures in northern Utah are not random in time and the average values are misleading at best. Rather, preliminary analyses suggest they exhibit several harmonics or cycles of different lengths. This is revealed as distinct periods of hot summers as well as sections of time with cooler summer temperatures. In addition, initial phase space analysis indicates summer temperatures have shifted to a hotter value starting about 1985. The “hot” sequences of summers are getting hotter in this rapidly growing region of the west. Current Earth atmosphere system models insist upon significant increases in summer temperature in the large region of the western US, with Utah near the center of the zone. So the hot episodes will grow even warmer. This has large implications for water usage in the region, since transpiration responds to saturation deficit, which increases with temperature in a nonlinear fashion.

The first objective is to document the cyclical nature of summer temperatures, and the resulting periodic sequences of “hot” summers. Also, use complex system methods to quantify if there is a general change in summer temperatures, and when it began.

The second goal is to use knowledge of the response of stomatal conductance of irrigated turfgrass to saturation deficit of the air, to construct a “diagnostic” version of the Penman-Monteith equation. This can be tested with previous ET measurements made at Utah State University over irrigated turfgrass. This model can be combined with summer temperature and humidity values to quantify the changes in water used by irrigation of landscapes in response to the cycles of summer heat. Using climate model projections, likely future hot summer episodes can be estimated, to reveal expected cycles in water demand. Unlike traditional averages which are misleading in this case, this approach leads to the form of information that is needed to assess future strategies in dealing with changes in water resources.