Improving Binational Water Management Policy through Science Presented by Suzanne Tillery, PhD - Hydrologist, U.S. IBWC May 31, 2017

North American Southwest Water is vital but disappearing resource In the North American Southwest, water is a vital but disappearing resource. 

Increasing population and irrigation demands Along international border of the Rio Grande between the U.S. and Mexico Puts strain on already sparse resource Increasing population and irrigation demands along the international border of the Rio Grande are putting a strain on an already sparse resource.

Water management officials along U.S. / Mexican border Using binational water management policies over 100 years old

To more efficiently manage water use in this region New approaches using science & technology are needed To better allocate Rio Grande water to meet needs of both countries In order to more efficiently manage water use in this region, new approaches using science and technology are required to better allocate water to the two countries.

International Boundary & Water Commission U.S. Section Mexican Section Provides binational solutions to issues related to treaties Boundary demarcation Ownership of water Sanitation Water quality Flood control

Water along border is shared in accordance with U.S. – Mexico 1944 Water Treaty for the Utilization of Water of the Colorado and Tijuana Rivers and of the Rio Grande (Treaty series 994, 1946)

Treaty Summary: Article 4 In accordance with Article 4 of the 1944 Water Treaty, one-third of the flow reaching the Rio Grande from six (6) named-tributaries in Mexico is allotted to the U.S., while the other two-thirds are allotted to Mexico. Over a five-year period, the one-third delivered to the U.S. “…shall not be less, as an average amount in cycles of five consecutive years, than 350,000 acre-feet (431,721,000 cubic meters) annually…Over the five-year period, a minimum of 2,158,605,000 cubic meters (1,750,000 acre-feet) should be delivered to the U.S. (Estimated percents 1950-2013)

Since 1992 Four five-year cycles have ended in deficit In the past, there have been five-year cycles in which the delivery of water to the U.S. from the six named-tributaries has fallen short of the 431,721 thousand cubic meters (TCM) annual average target delivery rate described by the treaty. The four most recent five-year cycles that ended in deficit occurred during the cycle periods of 1992 through 1997, 1997 through 2002, 2002 through 2007, and 2010 through 2015. Current cycle already in deficit

Recognizing these measures and present hydrologic conditions By not meeting the obligation, various measures have been taken to satisfy the deficit Recognizing these measures and present hydrologic conditions More proactive options and measures are being analyzed To meet treaty obligations To ensure U.S. receives all water allotted to it in timely and predictable manner By not meeting this obligation, various measures have been taken in order to satisfy the five-year cycle deficits. Recognizing the measures taken to satisfy previous debts and present hydrologic conditions, more proactive options and measures are being analyzed to meet the obligations of the 1944 Water Treaty and ensure that the U.S. receives any and all water allotted to it in a timely and predictable manner.

U.S. Section of IBWC developed a RiverWare model based on treaty requirements Explore new and innovative delivery options That maintain spirit of the treaty While ensuring compliance and availability of water to upstream users A RiverWare™ model was developed based on the requirements as defined by the 1944 Water Treaty to explore new and innovative delivery options that maintain the spirit of the treaty while ensuring compliance and continued availability of water to upstream users.

Model Components Named Tributaries Dams on Tribs Francisco I Madero La Boquilla Luis L Leon Centenario San Miguel La Fragua Venustiano Carranza Las Blancas El Cuchillo Marte R Gomez International Reservoirs Tributaries below Falcon The six named-tributaries are allocated downstream from Presidio, Texas-Ojinaga, Chihuahua. Therefore, the area specified for this modeling effort is from the Rio Grande near Presidio, Texas-Ojinaga, Chihuahua, to the Gulf of Mexico, near Brownsville, Texas-Matamoros, Tamaulipas. There are multiple dams in Mexico that control releases to the Rio Grande, as indicated by the blue triangle shapes in the figure. Along the Rio Grande, there are two International Reservoirs that straddle the border between the U.S. and Mexico: Amistad Reservoir and Falcon Reservoir, as indicated by the pink triangle shapes. The tributaries below Falcon Reservoir that are identified in the 1944 Water Treaty and normally allotted 100% to Mexico are Rio Alamo and Rio San Juan.

Model Methodology Monthly RiverWare model Evaluate alternate sources of water to meet delivery requirements Consider effects on Mexican reservoir system and International reservoirs Mexico and U.S. must agree on deliveries other than 1/3 of Named-Tributaries A RiverWare™ model was developed to represent the area from the Rio Grande near Presidio, Texas-Ojinaga, Chihuahua, to the Gulf of Mexico on a monthly basis to evaluate alternate sources of water that Mexico could deliver to the U.S. in order to meet the minimum annual average volume of water identified in the 1944 Water Treaty. Each of the relevant hydrologic items in this region are included in the model, plus a number of options that may be employed to modify the amount of water delivered from Mexico to the U.S. The resulting effects of these options on the Mexican reservoir system and the International Reservoirs, Amistad and Falcon, are provided in order to evaluate the effectiveness of the various options. Mexico and the U.S. must agree on deliveries for any sources other than the treaty-specified one-third of the six named-tributaries. The Accounting option of RiverWare™ was used to distinguish the different types of water and the ownership of the water.

Case Study A variety of scenarios were simulated with the RiverWare model On historic five-year cycles that ended in deficit To consider the effects of adjustments on Delivery volume Storage in Mexican and International reservoirs One of the case studies is presented as an example Two five-year cycles 2010-2015 2015-2020 Study done near the end of the 2010-2015 cycle which was ending in deficit Shows the adjustment settings And the results A variety of scenarios were simulated with the RiverWare™ model on historic five-year cycles that ended in a deficit (i.e. did not meet the treaty required target) to consider the effects of adjustments on both the change in delivery volume and changes in the Mexican reservoirs.  

Delivery Adjustment Types Additional water from Mexican reservoirs Deliver a specified volume to U.S. when below target delivery and conservation level is above a specified percent When the storage volume in a Mexican reservoir is at a user-specified percent of its conservation level, deliver a user-designated volume of water.

Delivery Adjustment Types Increase U.S. share of Six Named- Tributaries Mexico normally delivers 1/3 of flow Deliver a greater share to the U.S. In accordance with the 1944 Water Treaty, the U.S. receives one-third of the flow in the six named-tributaries. Increase the delivered water to the U.S. from Mexico's two-thirds share of the six treaty tributaries when the U.S. agrees to accept such water and can put it to beneficial use. The U.S. and Mexico would agree to credit deliveries from this source to Mexico's five-year cycle obligation

Delivery Adjustment Types Water from Rio Alamo and Rio San Juan Typically Mexico receives 100% of this water Provide some % of this flow when reservoirs are above their conservation level Typically, Mexico receives 100% of the flow from the Rio Alamo and the Rio San Juan, which are located below Falcon Reservoir. The adjustment option for these tributaries, based on article 9(e) of the 1944 Water Treaty, provides for some percentage of this flow to be delivered to the U.S. from Rio Alamo when Las Blancas storage is above its conservation level, or from Rio San Juan when Marte R. Gómez storage is above its conservation level. However, this delivery is contingent on Texas Commission on Environmental Quality (TCEQ), the primary stakeholder in Texas, being able to use and accepting the water delivered.

Delivery Adjustment Types Increase U.S. share of Unmeasured Tributary Flow Normally split 50/50 Mexico could give some of it’s share to U.S. Normally, positive unmeasured tributary inflow is split 50/50 between the U.S. and Mexico. Provide a portion of Mexico's share of this unmeasured tributary flows to the U.S. The U.S. and Mexico would agree to credit deliveries from this source to Mexico's five-year cycle obligation.

Delivery Adjustment Types Transfer of Storage in International Reservoirs Specify amount of water in each reservoir to transfer to meet deficit Usually at end of a cycle in deficit Ownership of storage may be transferred from Mexico to the U.S. in the International Reservoirs, Amistad and/or Falcon. Article 9 of the treaty and Minute 234 permit use of other sources: Additional reservoir releases Rio San Juan Greater than 1/3 at named tribs. Reservoir transfer

Case Study Review results A variety of scenarios were simulated with the RiverWare™ model on historic five-year cycles that ended in a deficit (i.e. did not meet the treaty required target as shown in the figure) to consider the effects of adjustments on both the change in delivery volume and changes in the Mexican reservoirs. One case study is reviewed here.  

Shows the accumulated delivery for each cycle, and how adjustments helped meet the deficit. The sinuous black-dashed line represents the delivery target with seasonal adjustments.

RESULTS: Shows the accumulated values for each of the adjustment types RESULTS: Shows the accumulated values for each of the adjustment types. The green dashed line in the first cycle represents adjustments agree to during that cycle.

RESULTS: Shows the accumulated values for each reservoir in the additional MX reservoir release adjustment type

RESULTS: Shows the values for Luis Leon reservoir as result of the additional MX reservoir release adjustment

RESULTS: Shows the accumulated values for named-tributary flows and adjusted flows

RESULTS: Shows the accumulated values for unmeasured tributary flow

Conclusions The RiverWare model can Act as an invaluable tool for evaluating alternate scenarios of water delivery in a complex system Based on science and technology While providing potential consequences resulting from the various adjustments Working together, the U.S. and Mexico can jointly agree to adjustments that are acceptable to both countries This model can act as an invaluable tool in providing the U.S. and Mexican Sections of the IBWC a method based on science and technology to evaluate a complex system and consider the results of the proposed action. Working together, the U.S. and Mexico can jointly agree to adjustments that are acceptable to both countries to meet the target delivery specified by the 1944 Water Treaty.

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