Interbasin Transfers and Water Risk in the United States

2018-10-31T15:53:44Z (GMT) by Kerim Dickson
Some parts of the U.S. have strained or insufficient local water supplies to meet the demands of population, industry and agriculture located in the region. Some areas with insufficient water supply have long implemented measures to address the shortfall through transferring water from other basins. New York City obtains almost 97% of its water and Los Angeles over 90% from interbasin transfers (IBTs).<br>With climate change affecting precipitation and temperature patterns across the U.S., coupled with growth in population and the economy leading to changes in demand, planning for risks to water supplies is critical to ensuring continued supply of water for all U.S. regions. Assessment of areas of high and low water risk can provide insights into potential changes in availability for existing supply, and aid in decision making for mitigating forecasted risks to local water supply. Implementation of IBTs historically has been one approach for addressing water supply risks.<br>The overarching goal of this research was to examine the role of IBTs for water resource supply and management in the U.S. Specific objectives were as follows:<br><div>1. Quantify the number of IBTs that exist at a defined hydrologic unit code (HUC) level in the U.S. and examine the distribution of IBTs and potential causes associated with any observed clustering of IBTs. <br></div><div>2. Characterize and classify IBTs, and examine the development drivers for a subset of IBTs in the U.S through sampling in different climate regions of the U.S.<br>3. Examine the water risks in the U.S. by county, considering both current and future conditions and accounting for natural water importation through streams and rivers, and consider the role of IBTs in mitigating these risks.<br>As part of the first objective, the definition of what constitutes a “basin” was required to assess man-made transfers that cross those basin boundaries. There are several definitions utilized by different states, with no federal definition. The most recent inventory of IBTs was conducted by the USGS in 1985 and 1986 using the HUC4 level. To build a new inventory of IBTs in the U.S., the National Hydrography Dataset (NHD) was utilized, combined with the Watershed Boundary Dataset (WBD). Man-made transfers across basin boundaries at the HUC6 level were considered to be interbasin. Geographic Information Systems (GIS) analysis showed that as of 2016 there were 2,161 IBTs crossing HUC6 boundaries in the U.S. These were located across the country, although over 50% of those identified were located in Florida, Texas or North Carolina. Some clustering of IBTs was observed in various states and analysis of the clustering suggested a variety of reasons for IBT construction, including population, drainage and agricultural factors. However, the flow volumes associated with the IBTs identified could not be evaluated due to a lack of available data at both the state and federal level.<br>The second objective expanded upon this analysis, examining a subset of 109 (5%) of the identified IBT reaches within the various climate regions of the U.S. To characterize and classify the IBTs each was labeled as being near irrigated agricultural land, near cities, or rural for those not near either cities or irrigated land. IBTs in proximity to both cities and irrigated agricultural lands were given the designation city+irrigated agriculture. Selection of IBTs for this analysis was based on the approximate proportional distribution of the total number of IBTs within each climate region and included representation of IBT clusters identified as part of the first objective. The results of the analysis showed that there have been four major drivers behind the construction of IBTs in the U.S.: irrigation for agriculture, municipal and industrial water supply, commercial shipping or navigation, and drainage or flood management. The most common factor for IBT construction has been to enable drainage or flood management. IBT development for agricultural needs has also been prevalent. The majority of IBTs examined were constructed between 1880 and 1980, with peaks in construction occurring between 1900-1910 and 1960-1970. The case studies examined showed that drivers of IBT development evolved through history, reflecting the changes in U.S. and regional economies, populations and needs.<br>To examine the risks associated with the U.S. water supply a new Water Risk Index (WRI) was developed, building upon and advancing a prior risk analysis developed by Roy et al. (2012). The Roy et al. work utilized risk factors that focused upon local precipitation, demand and evapotranspiration, without examining the natural flow of water between counties. To produce the WRI the analysis utilized the 2015 USGS Water Use Report data and projected water use in 2050, assuming only municipal and domestic water demand and thermoelectric power water withdrawal demand would change over time as per Roy et al. (2012). To calculate the flow volumes for each county the Water Supply Sustainability Index (WaSSI) developed by the USDA Forest Service (Sun, 2008) was used. The WaSSI model allowed for the analysis to include changes in climate and related hydrology as well as the evolving water demand. The WRI calculated water supply risk for each county in the contiguous U.S. The WRI calculation includes comparisons of water withdrawal to local flow volume, the drought susceptibility during summer for both the present and future, the projected growth in water demand, and the proportion of groundwater use relative to total water demand. This risk index provides a scaled value system that provides context to each individual risk factor included. The results of this showed that while some counties are regarded as high or very high risk, there are significantly fewer than those identified by the Roy et al. (2012) analysis. A maximum of 36 counties were identified as high or very high risk within the scenarios examined as part of the WRI analysis, in comparison to over 400 in the previous analysis. The highest risk areas are located in the west, with most counties determined to be at very high risk located in California. Most of the counties with negligible risk are located in Montana and Wyoming, as well as Colorado west of the continental divide.<br>This research provides insights into locations within the U.S. that may have high risks to their water supplies, and into the role that current or potential IBTs can have to mitigate those risks. In addition, the methods developed can help support planners to identify low risk locations to examine for their potential to support IBT water supply solutions while accounting for the downstream impacts such diversions may cause. To ensure that the U.S. maintains a consistent and secure water supply all options must be considered for their viability, including the potential for moving water from where it is plentiful to areas it is not.<br> </div>