Drought monitoring is essential in the management of water resources, especially in underserved and arid areas such as the Navajo Nation. The DEVELOP team at NASA Ames Research Center examined methods for calculating Standard Precipitation Index (SPI) values foråÊthe Nation using NASA Earth observation data within ArcGIS, R, and RShiny.
The Navajo Nation
The Navajo Nation, located within the southwestern United States, is the largest Native American territory in the country in terms of land area and population (US Census 2010). Sitting atop the Colorado Plateau, the Nation is classified as arid to semi-arid, and contains a mix of topography, including high deserts and alpine forests. Historically, precipitation trends in the Navajo Nation exhibit two rainy seasons a year: one in the winter and one in the summer. The two rainy seasons are distinctly different in precipitation regimes andåÊdistributions. Winter brings even and low-intensity precipitation over large areas, whereas summer brings intense, often-localized precipitation over small areas (Crimmins 2013). The periodic oscillations between wet and dry seasons create a complex environment for monitoring drought and water resources.
Water in the Nation
Drought moved to the forefront of Navajo Nation concerns when a state of emergency due to drought was declared (Ahtone 2013). The Navajo Nation has a long and tumultuous history of fighting for water access rights within and along its boundaries. Recent litigation has been filed to address the Nation’s water-stricken state. Most notable examples involve disputes over rights to water in the San Juan River Basin, the Colorado River, and the Little Colorado River. Although the Nation won a settlement regarding rights to water in the San Juan River Basin (NNDOJ 2015), this victory does not provide enough water to sufficiently serve the entirety of the vast nation. Additionally, a lawsuit regarding rights to unallocated water in the Little Colorado River was initially settled, however the settlement was re-continued in court (NNDOJ 2015). Thus, legal battles for Navajo water rights are ongoing.
The increase of commercial use of Navajo water resources has decreased water availability for Nation residents. Peabody Energy, a southwest-based energy company and the largest private-sector coal company in the world, has been a dominant consumer of groundwater in the Nation since the 1960s. Until 2005, Peabody operated two coal strip mines within Navajo boundaries: the Black Mesa Mine and the Kayenta Mine. Peabody has also operated two associated power plants responsible for burning the harvested coal: the Mohave Generating Station outside the Nation, and the Navajo Generating Station inside Nation borders. Ironically, the Navajo Generating Station provides power for pumping water from the Colorado River, which is currently inaccessible to the Navajo Nation.
Currently, Peabody Energy utilizes vast amounts of water to transport and wash coal harvests, which places the Nation at a significant disadvantage in the spectrum of water availability. The Black Mesa Mine and Mohave Generating Station consumed 3 million gallons of water a day from the Navajo Aquifer until they were shut down in 2005 (Black Mesa Water Coalition 2015). The Kayenta Mine is currently operational and uses an annual amount of approximately 1,240 acre-feet of water, or 400 million gallons from the Navajo Aquifer water supply (U.S. Office of Surface Mining Reclamation and Enforcement). This translates to an average of 1.1 million gallons of water per day still being removed from the Navajo Aquifer, a crucial source of water for a potentially drought-stricken Navajo Nation.
As of 2011, water scarcity and a lack of infrastructure have rendered roughly 70,000 Navajo residents without access to potable water in their homes (U.S. Census 2000, NNDWR 2011). Community wells and water points for residential collection are configured to regulate water supply; however, the vastness of the Nation’s area proves to be a hurdle for residents far away from these posts. To address this issue, water hauling methods are commonly used by residents as a delivery technique; however, this method is economically unsustainable, and can cost as much as $43,000 per acre-foot of water compared to the $600 typically paid in surrounding areas. As a consequence, unregulated water sources that are easier to access are being used to supplement needs. These unregulated sources include livestock wells and springs, which are not monitored by the Navajo Nation Environmental Protection Agency, and are found to have high concentrations of heavy metals and bacteria such as E. coli (NNEPA). The Navajo Nation has a higher percentage of people lacking the basic necessity of water in their homes than any other region in the U.S. (NNDWR 2011). This lack of access places additional pressure on Navajo natural resource managers to provide solutions to a very complex issue.
The agricultural and ranching industries also have experienced severe impacts due to desiccated lakes and ponds intended for livestock and fishing, and this has increased dependence on groundwater sources. As Navajo Nation spokesman Erny Zah described in 2013, shallow wells designated for livestock may become the next option as a resource (Zah 2013). However, this solution may not be a viable alternative, as harmful trace amounts of arsenic and uranium in the topsoil can leak into these shallow wells, posing a serious risk to human and livestock health.
The most recent national climate change assessment found that native populations in the Southwest are among the most vulnerable groups to climate change due to the coupled effect of harsh climatic impacts and high rates of poverty (Garfin et al. 2014). The decline in rainfall and streamflow, along with a low water table, are added concerns for this vulnerable state. Given the severity of the current drought and the increasingly negative impacts of extreme changes in climate, it is critical that attention is given to monitoring and managing available water resources in this region.
Standardized Precipitation Index
Defining and characterizing drought can be a difficult task due to numerous existing definitions of this phenomenon. The Navajo Nation uses a well-known and internationally-used Standardized Precipitation Index (SPI) to monitor and characterize drought. The SPI serves to transform raw data into meaningful quantities to allow ease of monitoring, documentation, and comparison among data (Zagar et al. 2011, McKee et al. 1993). It is calculated by comparing a normal distribution of monthly accumulated rainfall to historical precipitation trends in the same region and time of year. This ultimately determines specific SPI values, which indicate anomalous wet or dry time periods.
An SPI can be calculated for varying timescales, depending on user needs. For example, a six-month SPI has been shown to be a sound indicator of meteorological drought, while a 24-month SPI has been shown to be a sound indicator of groundwater recharge (Zagar et al. 2011).
The Navajo Nation currently uses a six-month SPI calculated by the Western Regional Climate Center (WRCC). Although this is a standard approach to drought monitoring, this specific calculation does not account for the climatic intricacies of the Nation. The WRCC calculates SPIs for areas of the U.S. Climate Divisional Dataset, a state-based divisional system that does not take the Nation’s political boundaries into account. This results in the majority of the Nation falling into one climate division, therefore receiving only one SPI. In comparison, the state of West Virginia, which has roughly the same area as the Navajo Nation, has six divisions, and receives six SPI values. This allows for more specific drought monitoring of the area.
To improve drought monitoring capabilities, the DEVELOP team at NASA Ames Research Center in Mountain View, CA utilized NASA Earth observation data to examine three separate approaches for calculating SPI values more specific to the Navajo Nation. This project partnered with the Navajo Nation Department of Water Resources: Water Management Branch, and the Navajo Technical University, to create a database of drought-related NASA Earth observation data. This project also investigated methodologies for creating 1) a rasterized (gridded) SPI-calculating geoprocessing package and 2) a tool utilizing R language statistical and raster modules wrapped in aåÊuser interface coded usingåÊShiny by RStudio.
To compile a modern precipitation database, the DEVELOP team used data from NASA’s Tropical Rainfall Monitoring Mission (TRMM) and Global Precipitation Monitoring (GPM) satellites, resulting in rasterized monthly accumulated precipitation from June 2001 to the present. The team also compiled a historical database of monthly precipitation from 1901 to 2000 from the Parameter-elevation Relationships on Independent Slope Model (PRISM), created by the PRISM Climate Group at Oregon State University.
The DEVELOP team examined three methods of creating raster SPI maps using Esri’s ArcGIS, geographic information system software, in combination with Microsoft Excel, a Disk Operating System (DOS) program, and the statistical program R.
The first method involved transferring data from Excel to ArcGIS. In Excel, a function consisting of all required equations for calculating an SPI is built within a spreadsheet. Gridded monthly precipitation values are imported into this Excel sheet, and the calculated SPI values are then outputted and transferred back into ArcGIS to create an SPI map. Benefits of using Excel include user-familiarity with the program, and flexibility in formatting the data output from ArcGIS. However, the need for data manipulation by the user leads to more room for error, and there does not appear to be a way to easily automate the transfer of data from ArcGIS to Excel and vice versa. This is an important issue to consider when working with large amounts of data.
The second method evaluated in this project, the DOS program, was written by the National Drought Mitigation Center, and has the capacity to export precipitation data from ArcGIS. However, data formatting required for the program to operate is very specific, and exported data must be manipulated in Excel prior to executing the program. While this method is beneficial in that the authors of this DOS program currently collaborate with the Navajo Nation, the amount of data manipulation needed to calculate the SPI for each cell is discouraging and is prone to human error.
The final method examined was the integration of an R module into ArcGIS. This method also requires a specific format of precipitation data; however, a program can be written within R to transfer the original data into the format required by the module. This eliminates the need for user manipulation of data. Additionally, a pre-written function currently exists for calculating SPI values within R.
According to this investigation, R is the most viable option for SPI calculation and has potential in interfacing with ArcGIS. At ArcGIS 10.3.1, users can download the R-ArcGIS åÊ”r-bridge” on GitHub which will allow R scripts to be run within ArcMap. åÊArcGIS Desktop 10.3.1 is the most recent version of the program at the time of this article and is the latest version used by Navajo Nation project partners. As there is a pre-existing function for calculating SPI within R, and as R is specifically a statistical computation program, it has the highest potential among the three options evaluated for robust and complex statistical calculations.
The Navajo Nation Department of Water Resources will be able to analyze the results from this study and assess which methodology best suits their needs. The precipitation databases created will allow precipitation trends within the Nation to be tracked with greater spatial resolution than previously available. By utilizing NASA Earth observations, Navajo Nation water managers and decision-makers will have the ability to identify the most drought-impacted areas, and have greater knowledge to inform drought mitigation, water management, and policy decisions. The use of NASA Earth observation data may help mitigate drought severity for Navajo Nation residents, and assist policymakers in better understanding future needs and decision-making processes related to water resource management within the region.
This material is based upon work supported by NASA through contract NNL11AA00B and cooperative agreement NNX14AB60A.
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