*1.2. Regional Irrigation Efficiency*

Precautionary measures for ensuring future supply often focus on reducing future demand by increasing IE. IE's formal definitions are explained in Appendix A. IE policy aims to maximize the consumptive use portion of water withdrawals to obtain "more crop per drop" [21]. The essential assumption about irrigation efficiency is that by reducing the portion of non-consumptive use (e.g., surface water diverted that percolates back to groundwater) of total diversions, water is being conserved. The investment in advanced irrigation systems leads IE's increase, which keeps the root zone saturated with limited irrigation. The saving of water at the field scale impacts the whole socio-hydrology system. The water saved results from IE policy implementation; however, it is not aligned with regional sustainability in agriculture and water. Furthermore, IE's investment and its impacts regionally are particularly of interest to agricultural development in dry regions with limited available water supplies. Agricultural water demand is affected by various factors. IE policy achieved in one study performed in New Mexico led to a reduction in water applied per hectare but increasing water depletion based on various basin data analyses [22].

Achieving real water savings requires understanding institutional, technical, and accounting measures that accurately track and economically reward reduced water depletions. Conservation programs that target reduced water diversions or applications

provide no guarantee of saving water. The systematic understanding leads to cooperation on regional sustainability goals from diverse water users.

Implications of IE policy should be calculated in a broader context reflecting a mathematical perspective as well as a systematic perspective that includes uses that are traditionally classified as nonbeneficial. The mathematical perspective reflects IE policy's performance based on IE's definition; systematic perspective reflects the changes in the system's components affected by IE policy. Encompassing water managemen<sup>t</sup> calls for understanding the interconnections between potential solutions and systematic consequences [17].

This paper aims to examine the utility of system dynamics modeling by analyzing the costs and benefits of implementing an IE policy. To address the overall effect of IE policy on agriculture sustainability and water sustainability, this study couples the dynamics of the agricultural hydrologic cycle, irrigation management, population, and economic development with interconnected feedbacks through system dynamics modeling. IE policy's impact is investigated with three climate scenarios through system dynamics simulations within New Mexico's LRG region from 1969 to 2099. It is hypothesized that system dynamics modeling would provide useful insights for informing IE policy. Specifically, it is hypothesized that unintended consequences of IE policy would be illuminated, such as decreased hydrologic connectivity, groundwater depletion, and negative economic impacts. Simulation results report the model's pertinent performance measures of irrigated agriculture and local economies in response to the policy scenario.

### **2. Overview of the Research Area**

The Rio Grande River flows through three states (Colorado, New Mexico, and Texas) and forms the border between two countries (the Republic of Mexico and the United States of America). The Rio Grande has its headwaters in the San Juan Mountains of Colorado and terminates in the Gulf of Mexico. The river forms a valley through New Mexico; a large majority of the land use in the valley consists of irrigated agriculture [23]. The valleys surrounding the Rio Grande in New Mexico are categorized into three regions: upper Rio Grande, middle Rio Grande, and lower Rio Grande (LRG). The latter is focused on here (Figure 1). The LRG planning region includes all of Doña Ana County, and the total area of the planning region is 3814 square miles (9878 square km). Agriculture is the predominant land use adjacent to the Rio Grande in this area [23]. The climate is semiarid, and annual precipitation ranges from 8 to 20 inches, depending on topography. The majority of precipitation falls as rainfall during the monsoon season. Geology surrounding the LRG consists of 150 to 400 feet of alluvium in which unconfined aquifers are highly connected with the river [24]. Fuchs et al. [25] stated that groundwater storage change is positively correlated with surface water use in the LRG region. The LRG region is located downstream of the Elephant Butte Reservoir, which supplies surface water to irrigators and happens to be experiencing a megadrought (drought persisting more than 20 years) [26].

The majority of water diverted from the Elephant Butte Reservoir is for agricultural use (87%), with only a small amount diverted for residential use [27]. Groundwater pumping is an indispensable supplement to surface irrigation supply impacting Rio Grande's managemen<sup>t</sup> operation. The current operating agreemen<sup>t</sup> since 2008 among the Elephant Butte Irrigation District, El Paso County Water Improvement District, and the U.S. Bureau of Reclamation has been to release 150,000 additional acre-ft from the reservoir in full-supply years to meet downstream delivery requirements of Texas and Mexico [28]. Because agriculture in this region plays a central role in the local economy, stakeholders' perception and the economic value of crop yields jointly determine the water demand resulting in a low elasticity for water demand [29]. Water delivery requirements not met by surface water are supplied by groundwater. During drought, groundwater is presumably the crucial alternative water source.

**Figure 1.** Generalized map of the lower Rio Grande region, its location in the US, and land cover.

The LRG region's recorded population in 2010 was 209,000, which is continuing to increase [30]. The rising municipal water demand to match population growth creates the risk that water rights will be transferred from agriculture to urban areas, compressing the available agricultural water supply. New Mexico has been leading national pecan production after 2018. Hurricane Michael severely impacted the Georgia pecan industry [18]. Of the state's 92 million pounds of pecans in 2017, Doña Ana county produced 66.9 million pounds. Pecan orchards comprise over 30% of the LRG region. During an irrigation season, pecan orchards need 3.6–6.6 ft irrigation in the southwestern United States [19,20]. Doña Ana also leads statewide pasture production [21]. A sufficient water supply is required to ensure profitable agricultural production, preventing substantial reallocation of water from agriculture to other sectors [31].
