Middle Rio Grande Water Budget
/ Rio Grande Compact
A water budget is a quantitative accounting of where water comes from and where it goes. It can be future looking as plans or past-looking as how we've performed. Like a financial budget it reflects a net gain or loss. Monitoring the water budget for the middle Rio Grande is crucial in understanding our ability to meet interstate compact obligations and to ensure we can provide water for future years and future generations
Posts - Any technical papers, data, opinions, announcements, etc. that relate to this Water Budget/Rio Grande Compact issue appear just below.
New Mexico has not met its obligations to deliver Rio Grande water downstream under the Rio Grande Compact among CO, NM, and TX. Over the past decade, our under-deliveries through the Middle Rio Grande to Elephant Butte have brought us from a substantial credit into a significant debit for the first time since 1990. If…Read More
The Rio Grande New Mexico Basin Study seeks to develop a credible, technically based model of the Rio Grande’s possible futures, in the environment of varied management and climate alternatives. When complete, the Study will provide a sound scientific basis for planning the Rio Grande portion of New Mexico’s water future. It is one of…Read More
The U.S. Bureau of Reclamation’s Rio Grande New Mexico Basin Study conducted a webinar meeting on April 22, 2020 for all of the program partners (as of that date). The meeting agenda topics included the partners’ Memorandum of Agreement (MoA) with the program, the intended governance structure for the program and a preliminary look at…Read More
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Description of the Issue
The Rio Grande Compact is a formal treaty among states, signed in 1939. It annually apportions Rio Grande water among the states of Colorado, New Mexico, and Texas, according to the measured quantity of natural flows. Each upstream state is required to deliver a certain amount of water to its downstream neighbor. There are several components associated with New Mexico's portion. They are:
Rio Grande Compact
The Rio Grande Compact requires that most water entering the Middle Rio Grande annually at the Otowi gage must be delivered through the Middle Rio Grande for use by New Mexicans and Texans living downstream of Elephant Butte Dam.
The compact caps the Middle Rio Grande's total depletion of Rio Grande flows due to all causes, human and natural. Human depletions include the net reduction in the river's flow caused by all diversions of water, including groundwater pumping from aquifers hydrologically connected to the river. Human depletions also include evaporation from water storage reservoirs. Natural depletions include water consumed by riparian areas adjacent to the river and evaporated from the river itself.
Middle Rio Grande Water Entitlements and Water Delivery Obligations
The required annual downstream delivery--to and through Elephant Butte Reservoir--is based on the annual flow entering the Middle Rio Grande as measured at the Otowi stream gage. The Middle Rio Grande is allowed to consume the difference between the annual water flow measured at the Otowi gage and the required annual downstream delivery. Tributary and arroyo inflows that reach the river within the Middle Rio Grande factor into this difference.
Annual stormwater runoff during the monsoon season is an important element of the Middle Rio Grande's legal water budget. Unlike inflows to the Middle Rio Grande at the Otowi gage, which establish the annual downstream delivery obligation and can be only partially used, tributary inflows to the river do not affect the delivery obligation while increasing the surface water supply and contributing to actual downstream deliveries.
Total annual consumptive water uses of surface water in the Middle Rio Grande that exceed the allowable difference between inflows at the Otowi gage and required downstream deliveries at Elephant Butte create an annual shortfall in downstream deliveries. This shortfall is called an annual compact debit. Conversely, if uses are less than the allowable amount, the result is an annual compact credit.
Cumulative debits less than 200,000 acre-feet per year are allowed but restrict storage of water in reservoirs constructed after 1929. Cumulative debits greater than 200,000 acre-feet violate the compact, which is both federal and state law, under the direct jurisdiction of the US Supreme Court.
Middle Rio Grande Water Budget
What is a water budget? It's conceptually the same as a financial budget. The Middle Rio Grande water budget compares the legally-available surface water supply (credits or income) with water depletions (debits or expenditures).
The Middle Rio Grande Water Supply Study was commissioned by the NM Interstate Stream Commission and the US Army Corps of Engineers in 1999 to develop an updated water budget for the Middle Rio Grande. The study, revised in 2004, compared the mean annual water supply that is legally available to be consumed within the Middle Rio Grande with the total depletion of water for year 2000 land use and groundwater development conditions.
The study results are illustrated below.
Green arrows illustrate the renewable components of the water supply that can be legally depleted in the Middle Rio Grande. Red arrows show total water depletions. The blue arrow shows the net supply, moving downstream from top to bottom.
The study concluded total depletions of the river's flows for year 2000 conditions exceeded the average annual legally available supply, 1950-2000, by 40,000 acre-feet per year. This is 6% of the study's quantification of total surface water depletions.
The bottom section of the graphic shows an average of 124,000 acre-feet of the Middle Rio Grande's legally available supply flowed annually into Elephant Butte Reservoir. However, reservoir evaporation losses averaged 164,000 acre-feet, creating an average excessive annual depletion of 40,000 acre-feet.
The yellow arrow indicates that groundwater pumping in 2000 was depleting the aquifer about 70,000 acre-feet per year. All depletions of the aquifer will, over time, deplete the river by the same amount. This study therefore indicated the gap between our water demand and our average available renewable supply was 110,000 acre-feet per year, which is 16%.
A group of academic and government experts working with the Middle Rio Grande Water Assembly prepared a detailed draft water budget in 2014. This budget relied on a very different technical approach but reached a similar conclusion.
Middle Rio Grande Water Depletions
The Middle Rio Grande Water Supply Study calculated the amounts of water depletions that would occur on average under year 2000 land use and groundwater development conditions. Human-caused and natural depletions were each about half of the total.
History of Compact Compliance
Each year, the Rio Grande Compact Commission calculates whether the total flow of the river delivered through the Middle Rio Grande for use below Elephant Butte Dam was greater or less than the annual delivery requirement. The graphic below shows the cumulative result since accounting began in 1940.
Is Compact Compliance Entering a Dangerous Period?
New Mexico in 2019 incurred the first significant cumulative shortfall in its deliveries of water through the Middle Rio Grande to Elephant Butte Reservoir since 1990. The cumulative debit is -37,000 acre-feet. The trend since 2011 is bad.
If the Middle Rio Grande incurs another net debit in 2020, urgent action will be appropriate to prevent the cumulative debit from exceeding the maximum debit explicitly allowed by the compact.
As long as annual deliveries of water through the Middle Rio Grande comply with the Rio Grande Compact, Middle Rio Grande water uses are secure and protected from the ongoing US Supreme Court litigation focusing on the Lower Rio Grande below Elephant Butte Dam. Texas and the United States are both suing New Mexico regarding its depletion of surface water below Elephant Butte Dam and New Mexico has asserted counterclaims pertaining to unregulated groundwater pumping impacts and use of federal project water.
In the event Middle Rio Grande water deliveries violate the Rio Grande Compact's explicit provisions, the litigation will no doubt engulf the Middle Rio Grande and perhaps the Upper Rio Grande in New Mexico as well.