Potential impacts on Colorado Rocky Mountain weather due to land use changes on the adjacent Great Plains

Abstract

Abstract. Evidence from both meteorological stations and vegetational successional studies suggests that summer temperatures are decreasing in the mountain-plain system in northeast Colorado, particularly since the early 1980s. These trends are coincident with large changes in regional land cover. Trends in global, Northern Hemisphere and continental surface temperatures over the same period are insignificant. These observations suggest that changes in the climate of this mountain-plain system may be, in some part, a result of localized forcing mechanisms. In this study the effects of land use change on the northern Colorado plains, where large regions of grasslands have been transformed into both dry and irrigated agricultural lands, on regional weather is examined in an effort to understand this local deviation from larger-scale trends. We find with high-resolution numerical simulations of a 3-day summer period using a regional atmospheric-land surface model that replacing grasslands with irrigated and dry farmland can have impacts on regional weather and therefore climate which are not limited to regions of direct forcing. Higher elevations remote from regions of land use change are affected as well. Specifically, cases with altered landcover had cooler, moister boundary layers, and diminished low-level upslope winds over portions of the plains. At higher elevations, temperatures also were lower as was low-level convergence. Precipitation and cloud cover were substantially affected in mountain regions. We advance the hypothesis that observed land use changes may have already had a role in explaining part of the observed climate record in the northern Colorado mountain-plain system.

Problems in evaluating regional and local trends in temperature: an example from eastern Colorado, USA

“…There was evidence of a sub-regional cooling in autumn maximum temperatures, as five stations had significant decreasing trends, whereas only two stations had significant increasing trends. These results are consistent with a growing body of literature suggesting that local-and regional-scale irrigated agriculture can have a summer and early autumn cooling effect on climate for the region (Segal et al, 1988(Segal et al, , 1989 and over adjacent regions (Stohlgren et al, 1998;Chase et al, 1999). Local variation made it difficult to portray a regional trend in most climate parameters, and thus the simple climate projection from a century ago of a regional temperature trend would not have been of much value at a local level.…”

“…Temperature increases in rapidly urbanizing Fort Collins were typically three times higher than in rural sites (Table III), exaggerating the regional trend Owen 1998, Gallo et al, 1999). Likewise, local land-use effects from irrigated agriculture have been linked to a cooling effect in the summer (Stohlgren et al, 1998, Chase et al, 1999. Present GCMs lack the ability to simulate the complex climate patterns and anomalies in the simple topography but heterogeneous landscape of eastern Colorado.…”

“…How can one station (Holly) have significantly more days below freezing, while neighbouring sites have significantly fewer days below freezing? These results strongly suggest that regional trends can be seasonally variable, and local forcing from land-use change (Stohlgren et al, 1998;Chase et al, 1999), and other highly localized unknown causes, can override regional and global forcing.…”

Impact of agricultural practice on regional climate in a coupled land surface mesoscale model

“…In a regional modeling study, Segal et al [1998] found that irrigation resulted in an increase in precipitation in nonirrigated areas but did not produce any new rainfall areas. Similarly, Chase et al [1999], in a study corroborated by historical data, found that irrigation in the northern Colorado plains impacted climate in the plains and in the adjacent mountains. Other management factors, such as nitrogen addition and harvest timing, may also be important but have not yet been explicitly addressed.…”

Vegetation greenness impacts on maximum and minimum temperatures in northeast Colorado

“…Previous studies of the temperature regime in eastern Colorado include Kittel (1990), Segal et al (1988Segal et al ( , 1989, Stohlgren et al (1998), Chase et al (1999), Doesken (2000) and Pielke et al (2002).…”