Boise State University1, University of California, Santa Barbara2
Zeynep Hansen*, Gary Libecap+, Scott Lowe*#, Wenchao Xu*
*Boise State University; +University of California, Santa Barbara
# Corresponding Author: email@example.com; 1910 University Drive, Boise, ID, USA; +1 208-426-5439
Theme area: Sub-theme 17: Climate change, impacts and adaptation (Special Session 8: Agricultural Research)
This paper addresses the long-term impacts of climate change and water supply conditions on agricultural decisions in the arid western United States. By accounting for the inter-annual volatility of total water supply and the in-season distribution of stream flow observations, and by controlling for the time-varying development of water rights that govern water use and allocation, we attempt to disentangle the impacts that climate change and water supply factors have on the county-level agricultural activities, such as total acreage, farmland value, and crop failure, all measured over the past 100 years in Idaho.
Most of the previous studies of the impacts of climate change on agriculture have focused on the intra-season precipitation and temperature effects. Although this is relevant for the more humid East, these measures are less useful for agricultural production in the arid West. Specifically, irrigators in the snowmelt-dominated basins in the arid West are more dependent on pre-season snowpack, as 50%-80% of the total irrigation water in the North American West results from snowmelt -driven runoff in spring, which feeds irrigation throughout the growing season (Stewart et al. 2004).
In this paper we will address how inter-annual water supply volatility impacts farmland outcomes. In most of the West, these changes include trends toward earlier snowmelt-driven streamflows (Cayan et al. 2001; Stewart et al. 2005) that are more pronounced at lower elevations (Regonda et al. 2005), reduced late-season flow and increased early-season flow (Stewart et al. 2005). The most significant impact of a general warming was found to be a large reduction in mountain snow pack and a substantial shift in streamflow seasonality, so that by 2050, the spring streamflow maximum will arrive one month earlier in the year (Barnett et al. 2005). Since climate change reflects a long-term trend and adaptation and mitigation efforts take time to show their effects, it is necessary to investigate the impacts of climate change and water supply over a longer time span. Thus, utilizing historical agricultural (census) data will provide insight on what some of the future outcomes may be.
In Idaho, as in many agricultural areas in the arid west, measures of precipitation occurring prior to the growing season are second-best approximations of total water availability when compared to measures of snowpack or actual streamflow measures during the growing season. Yet snowpack is not a good measure of water supply. Studies have suggest that automated, fixed point, in situ snow measurements, which are the norm at many observation sites and are used for hydroclimatological analyses and decision making, consistently over- or under-represent the actual snow depth (Neumann et al., 2006). In addition, in many cases the snowpack occurs at great distances from the farms that will eventually utilize the irrigation water resources, and thus care has to be taken in matching the farm with the hydrologic basin which will eventually provide the water resources.
A number of institutional factors further complicate this analysis, most notably the priority of water rights in respective water types (eg: surface, ground, etc.). Water rights priority systems were established to assign a hierarchy of access to the water resources, and to prevent or resolve conflicts.
A similar advancement, with regards to accessing water, evolved with the electrification of the farm: Electrification enabled lands that consistently lacked surface water access and lands that regularly lacked surface water allocations to now have access to water. Groundwater pumping is expensive, and is a backstop for surface water use -- but it is more reliable and consistent, particularly for the junior irrigators that are more likely to be curtailed in any given year.
In order to address these complexities, we integrate county-level agricultural census data with water rights data and water supply indices. Because of the heterogeneity of surface and groundwater resources across the state, we disentangle both the composition of water sources for each county and the priority hierarchy for those counties. For our water availability measure, we use measures of the total water available during the growing season, as well as observed streamflow measures for the main water source for the four year moving average prior to the census year (for each county).
We use the water rights and water supply data together with agricultural census measures of farmland values, total acreage, and failed acreage in order to address the long-run impacts of water supply availability on agricultural outcomes. We hypothesize that farmland value and total acreage should increase with increases in water supply availability, but decrease with the vintage of the water supply -- particularly for the surfacewater dependent counties. We hypothesize the opposite effects for failed acreage.
Future analyses of the impacts of climate change on agricultural in arid regions should focus less on precipitation and more on the timing and runoff characteristics of the precipitation. Rain-on-snow events that reduce the snowpack could be detrimental to agricultural operations, as they rely more on the late-season availability of the snowpack runoff as opposed to aggregate. Similarly, these results may have implications for future adaptation or behavioral modifications that result from increased water trading or the use of water markets.
Barnett, T. P., J. C. Adam, and D. P. Lettenmaier (2005), Potential impacts of a warming climate on water availability in snow-dominated regions, Nature, 438(7066), 303Â–309.
Cayan, S., A. Kammerdiener, M. D. Dettinger, J. M. Caprio, and D. H. Peterson (2001), Changes in the onset of spring in the Western United States, Bulletin of American Meteorology Society, 82(3), 399Â–415.
Neumann, N. N., C. Derksen, C. Smith, and B. Goodison (2006), Characterizing local scale snow cover using point measurements during the winter season, Atmosphere-Ocean, 44(3), 257Â–269.
Regonda, S. K., B. Rajagopalan, M. Clark, and J. Pitlick (2005), Seasonal cycle shifts in hydroclimatology over the Western United States, Journal of Climate, 18, 372Â–384.
Stewart, I. T., D. R. Cayan, and M. D. Dettinger (2005), Changes towards earlier streamflow timing across Western North America, Journal of Climate, 18(8), 1136Â–1155.