1. Introduction
Over 50% of the world’s freshwater resources for human use and consumption rely on river discharge that can be greatly impacted by long-term changes in precipitation and temperature such as those caused by climate change, particularly in snow-dominated regions [
1]. Much of the western United States depends on precipitation falling in the winter in mountainous regions as snow and subsequently released slowly as snowmelt throughout the following spring and summer seasons. However, long-term changes in temperature and precipitation are already affecting these crucial water resource systems by decreasing the maximum snowpack accumulation, shifting the timing of runoff to arrive earlier, and impacting the volume of river discharge [
2,
3] with changes amplified by a lack of reservoir storage [
4]. More specifically, snow-dominated basins in the mid-high latitudes are the most vulnerable to the impacts of warming climates where maximum runoff is expected to arrive one month earlier by 2050 in the western United States [
1].
For example, the Rio Grande and its tributaries are increasingly becoming water stressed due to the warming climate and the increasing demand from users in Colorado, New Mexico, Texas, and Mexico [
5]. Rio Grande streamflow is vulnerable as it largely depends on snowpack conditions which are projected to decrease and melt earlier in the future [
6,
7,
8,
9]. This surface water resource must serve industrial, tourist, residential, agricultural, ecologic, and economic needs in the USA (e.g., Colorado, New Mexico, Texas) and Mexico. Under current climate conditions, New Mexico does not have water to spare between all users [
10].
Oftentimes, agricultural sectors are the largest users of water and face greater pressure to develop new water management strategies to help non-agricultural sectors cope with future water scarcity caused by warming temperatures and climate uncertainty [
11,
12]. In New Mexico in 2015, irrigated agriculture accounted for 76% of total water use, 53% from surface water and 47% from groundwater. Flood irrigation is used on 45% of all irrigated fields in New Mexico [
13].
Common water delivery systems for flood irrigation in New Mexico are
acequia networks which face many socio-environmental challenges. First introduced to northern New Mexico in the 16th century, acequias are gravity-driven water delivery networks and also serve as the basis of community-managed water governance systems [
14,
15]. While acequias have many beneficial hydrologic (e.g., aquifer recharge) and social attributes (e.g., water sharing) that foster resilience [
16], these ancient water systems still face the challenge of long-term, regional drought and difficult water policy [
17,
18]. Questions are continually raised at acequia irrigator meetings and posed to researchers regarding what the “right” management strategies are: Should we line the canals? Should we switch to drip? Should we pump groundwater? Irrigators find themselves stuck between cultural norms of propagating generational knowledge of traditional irrigation methods and pressures from decreasing water availability and outside agencies to modernize water delivery systems and maximize irrigation efficiency.
Agricultural irrigation practices involving surface water can cause percolation and groundwater recharge that significantly impact groundwater resources on regional scales [
12,
19,
20,
21,
22]. A study by Bouimouass et al. (2020) focused on the acequia counterpart in Morocco—
seguias—and concluded that flood irrigation of diverted surface water resulted in the dominant recharge process in mountain front landscapes [
23]. Other studies from large agricultural drainages in China found that approximately 70% of applied flood irrigation water in maize fields recharged the groundwater during the growing season [
24], and seepage from both irrigation canals and deep percolation (
DP) from irrigation contributed to more than 90% of total annual shallow groundwater recharge [
12]. Additionally, in a large traditional agricultural basin in Italy, irrigation water delivered through a system of canals provided 55 to 88% of groundwater recharge [
25].
DP is the amount of water that travels below the effective root zone (ERZ) that can potentially reach the shallow aquifer [
26]. One of our previous studies conducted in northern New Mexico showed that peak groundwater level response fluctuated up to 380 mm 8 to 16 h after the onset of flood irrigation [
27], where another estimated 16% of unlined irrigation canal flows seeped into the subsurface, causing the water table to rise 1 to 1.2 m [
28]. Additionally, annual shallow aquifer recharge ranged from 1044 to 1350 mm on a valley scale [
22]. In these cases,
DP from flood irrigation was a significant source of recharge to shallow groundwater.
DP below the vegetative root zone can provide very important hydrologic and ecosystem benefits in irrigated valleys of semiarid and arid regions.
Conversely, groundwater may display evidence of interactions with surface water. As irrigation water infiltrates into the shallow aquifer, this
DP can contribute groundwater return flows to the river. In northern New Mexico, this interaction is of particular interest considering
DP can serve as temporary subsurface storage which provides delayed return flow during low-flow periods [
22,
29,
30]. This serves as an important possible buffer for changing peak runoff timing associated with climate variability [
19].
Considering interacting surface water and groundwater as one resource is essential for optimal protection of watersheds, sustaining water resources, and furthering integrated groundwater management [
20,
31]. This is critical within irrigation districts that are increasingly relying on pumping groundwater for agricultural and municipal uses, which can lead to the disconnection of surface water and groundwater [
32]. More recently, groundwater recharge via flooding fields is becoming a more common conservation practice [
33,
34].
It is necessary to properly quantify aquifer recharge and foster an accurate understanding of
DP and surface water and groundwater interactions in water-limited regions [
35]. The water balance method is a technique commonly used to quantify groundwater recharge and characterize surface water and groundwater interactions [
19,
22,
26,
36,
37]. Components of the water balance are precipitation, irrigation water applied, runoff, change in soil water storage, and evapotranspiration, where
DP is unknown and calculated by the difference of these inputs and outputs [
26].
Our first objective was to characterize and compare surface water and groundwater interactions and shallow aquifer response to irrigation events in flood-irrigated forage grass fields located within the same irrigated valley in northern New Mexico by estimating
DP below the root zone with a water balance approach. Our second objective was to justify community-based adaptive management in the context of climate change by relating field-scale findings to regional climate change literature. The innovative approach of identifying tightly coupled objectives reflected the unique, tightly coupled natural and human irrigation system our study focused on. While cultivating a better understanding of available surface water resources is extremely important, irrigators and policy makers must also understand the effects of irrigation techniques on groundwater and surface water availability for downstream users [
31]. Previous studies have quantified and compared
DP across several crop fields, soil types, and valleys in northern New Mexico, USA [
22,
26]; however, more field observations of
DP are needed to expand these studies from field-scale to valley or regional scales. We hypothesized that: (1)
DP and total water applied and
DP and groundwater response would be positively related on both fields; and (2)
DP and total water applied would be significantly different across both study fields.
4. Discussion
Our results showed both fields have significant relationships between
DP and
TWA and between
DP and
TWA-∆S (
Table 6). One field, F2, exhibited significant relationships between
DP and irrigation duration (
Table 6), GWLF and
DP, and GWLF and
TWA (
Table 10). Antecedent soil moisture and soil conditions are particularly important factors to consider when discussing
DP. More irrigation water is needed to saturate the ERZ when antecedent soil moisture is low or at times of greater plant water use which results in potentially less groundwater recharge from a given amount of irrigation water applied.
DP was not significantly different when compared across both fields. The only significant differences when comparing irrigation events and
DP estimates across both fields were irrigation duration and the number of irrigation events (
Table 7). These results indicate that surface water and groundwater are tightly connected in this area, but variation in
DP and groundwater response exists between land managers and fields due to differing irrigation practices.
Although we only report data and results from the 2021 irrigation season, our data collection began in June 2020. When comparing monthly averages and totals for months with complete records for both 2020 and 2021, several patterns emerge regarding irrigation scheduling, average
∆S, and average
DP (
Table 5). F1 irrigation frequency tapered off toward the end of the irrigation season for both 2020 and 2021, while F2 irrigation frequency increased toward the end of the season. Average
∆S was constant between the two fields over both years of data collection, ranging from −3 to 2 mm with a notably large value for F1 in July 2021 (73 mm) as an outlier perhaps related to frequent rainfall that occurred around that time of year and uneven irrigation water application. On F1, the average
DP ranged from 2 to 29 mm over 2020 and 2021. Similarly, the F2 average
DP ranged from 1 to 32 mm. These patterns help validate our water balance results (
Table 2 and
Table 3) by demonstrating consistent and comparable water balance components and
DP estimates across both fields over 2020 and 2021.
Previous acequia research in northern New Mexico forage fields that also used water balance methodology to estimate
DP reflects similar results (
Table 11), illustrating that we appropriately captured acequia surface water and groundwater interactions and irrigation practices. Our results reflected the greatest
DP season totals (739 and 1249 mm) and the greatest percentage of
IRR that contributed to
DP (68 and 81%), critically filling in the range of possible seasonal
DP values and characteristics by refining our understanding of acequia irrigation-related recharge in the context of long-term field data collection in northern New Mexico.
Observations and projections of changing climate and snowmelt dynamics within the Rio Grande Basin—specifically the Upper Rio Grande headwaters region—are of particular interest to researchers and stakeholders due to the reliance of downstream users on snowmelt-dominated subbasins to meet water availability needs. For example, streamflow at Fort Quitman, Texas, USA has decreased 95% relative to the river’s native streamflow [
51]. In the Colorado River Basin, temperature-driven “hot droughts” have been connected to increased sublimation of snow which results in less runoff from a given snowpack [
52]. Similarly, the interannual variability of streamflow related to snow water equivalent (SWE) has decreased by 40% in the Upper Rio Grande Basin, indicating that the connection between peak SWE and runoff volume is substantially weaker [
7]. This drift between SWE and runoff is particularly critical because a large portion—50 to 75%—of the Rio Grande streamflow is sustained by seasonal snowpack accumulation [
53]. Through paleoclimate reconstructions published in 2017, researchers identified a 30-year declining trend in runoff ratio since the 1980s which appeared unprecedented in the context of the last 440 years [
54]. Observed, historical mean winter and spring temperatures have significantly increased in the Upper Rio Grande Basin [
7], and temperatures rose at an alarming rate of 0.4 °C per decade from 1971 through 2011, informing temperature predictions of a 2 to 3 °C increase in average temperature by the end of the 21st century [
8]. The SWE on April 1 has significantly decreased by 25% [
7], where the mean melt season snow covered area is predicted to decrease 57 to 82%, and peak flow is predicted to arrive 14 to 24 days earlier than usual [
6].
The combination of increasing temperature and more variable precipitation inputs are expected to create a decrease in summertime flows and increase the frequency, intensity, and duration of floods and droughts in the Upper Rio Grande Basin [
8]. Elias et al. (2015) found that total annual runoff volume of Upper Rio Grande subbasins and tributaries could increase 7% in wetter scenarios but decrease 18% in drier scenarios. In the Rio Hondo watershed, annual daily mean streamflow has significantly decreased 0.85% per year since water year 1976 [
55]. Another study found that the Rio Hondo baseflow, runoff, and streamflow have also significantly decreased since water year 1980 due to decreasing snowmelt rates [
56].
Decreasing surface water flows in the Upper Rio Grande region will have negative effects on acequia water availability for acequia communities in this region. A previous study conducted in the Rio Hondo Valley found statistically significant relationships between river and acequia flows [
57]. Similarly, spatial analysis of the normalized difference vegetation index (NDVI) found that the irrigated landscape within the Rio Hondo Valley expanded and contracted in response to wet or dry years, showing that irrigation intensity varied with available surface water [
58,
59]. Therefore, in the Rio Hondo Valley, acequia flow is directly related to river flow, and the variability of acequia irrigation intensity is apparent in wet and dry years. As a result, the irrigated landscape and acequia irrigation decrease as surface water resources decrease.
If surface water river flows continue to decrease, then acequia water availability and the acequia-irrigated landscape will decrease, as will regular
DP and groundwater recharge [
60]. As a mechanism that temporarily stores surface water in the subsurface which eventually returns to the river system as delayed return flow,
DP can serve as a very important buffer against climate change; however, mean recharge in Taos County first significantly decreased in 1996 [
61]. Baseflow is also an extremely critical element of the hydrologic regime in the Upper Rio Grande Basin where baseflow contributions account for 49% of total discharge upstream of Albuquerque, New Mexico [
56]. Surface water and groundwater connectivity is critical for continued baseflows, and acequia-related
DP and return flows play an important role in maintaining this connection. As climate change continues to negatively impact surface water availability and groundwater recharge in northern New Mexico and the Upper Rio Grande Basin, both acequia communities and the state of New Mexico will have to decide how to adapt to new climatic and hydrologic regimes.
When considering water use and management practices, either as a water manager or for modeling purposes, it is critical to determine the type and direction of adaptation (e.g., adaptation or maladaptation) occurring in response to climate change stressors [
62]. Maladaptive actions are enacted to prevent or reduce vulnerability associated with climate change but ultimately have adverse impacts or increase vulnerabilities in the same or related systems. Examples of adverse impacts include: (1) an increase greenhouse gas emissions; (2) a disproportionate impact on vulnerable populations; (3) high environmental opportunity costs; (4) reduced incentives to adapt; and (5) dependencies that limit future generations [
63]. Unfortunately, all too often, water management adaptation and governance strategies are maladaptive, such as water operations in Flint, Michigan [
64], water deliveries in California’s San Joaquin Valley [
65], and development in Australian coastal cities [
66].
Adaptive management practices are more prepared for climate change by incorporating flexibility and responsiveness into water management institutions and governance structures [
67,
68,
69,
70,
71,
72]. While some suggest doing this through intraregional contracts and mergers [
67], acequias have been doing this for centuries through a concept known as
repartimiento—the ability to employ flexible and dynamic water deliveries to distribute water as equitably as possible by sharing water shortages either within a single acequia or between different acequias throughout a given watershed. Cruz et al. (2019) documented this phenomenon by showing that the water available in acequias is directly correlated to the water available in the stream system [
57]. When not enough surface water is available to irrigate crops, landowners will typically irrigate a smaller parcel of their total crop land as opposed to the entire area. This shows the inherent adaptability embedded within traditional acequia irrigation frameworks that is and will continue to be crucial in the context of a changing climate, growing seasons, and streamflow regimes.
The flood irrigation regime these two fields and the greater Rio Hondo Valley—as well as other acequia communities—follow experience groundwater recharge benefits inadvertently associated with managed aquifer recharge (MAR). Recently, many research articles [
73] have featured different MAR techniques and pilot programs [
34,
74]. MAR is an approach used to replenish groundwater resources and is becoming more common in areas of heavy groundwater pumping and declining aquifer levels. There are many different techniques and objectives within this overarching mitigation approach. One promising approach that utilizes already existing infrastructure is applying MAR to irrigated agricultural lands, where surface water is applied over large areas as opposed to the more traditional MAR approach of facilitating high recharge at dedicated recharge sites [
34]. This form of MAR reduces costs associated with infrastructure, piping, and energy given the gravity-driven water distribution [
75]. This framework is naturally mirrored on a regional scale in northern New Mexico’s acequia networks. Acequia networks divert surface water through a system of (typically earthen) canals to fields for flood irrigation, where seepage occurs throughout time in the canals and application in the fields. Acequia irrigators greatly value these contributions to groundwater for the many environmental and water storage benefits the recharge provides (
Figure 4). Acequias are not without their challenges, but they can serve as a model for sustainable, integrated water management that implicitly employs MAR and welcomes groundwater recharge as a benefit rather than an inefficiency [
76].
Characterized by regular shallow aquifer recharge and flexible and dynamic water management that reflects equity and current environmental conditions, acequias offer several reasons why we should consider maintaining traditional irrigation systems in the face of climate change (
Figure 5). In times of reduced surface water availability, acequia irrigators only irrigate smaller parcels of their total irrigated land and typically invest in deep rooted, drought-tolerant crops that can persist through growing seasons without much irrigation water application. Acequia communities have followed this model traditional flood irrigation model and persisted through drought for hundreds of years in northern New Mexico. However, when thinking about the future, the question then becomes: How should acequia communities adapt to meet reduced surface water availability and changing streamflow regime challenges that the current prolonged and unprecedented drought presents if traditional acequia irrigation practices are no longer sufficient?
Traditional acequia operations are typically associated with resilience [
77], but many acequia irrigators and managers are unsure of how sustainable certain adaptations are moving forward (e.g., lining earthen irrigation canals, switching from flood to drip irrigation, greater reliance on groundwater pumping) and the implications of any detrimental effects on groundwater levels (i.e., lowering the water table) which are ultimately connected to surface water availability (
Figure 5). Lining irrigation ditches, switching from flood to drip irrigation, and supplemental groundwater pumping are commonly called into question by acequia community members which is why these adaptation strategies are highlighted in this paper. While these three strategies can be beneficial, lining ditches and drip irrigation reduce pathways for surface water to seep into the groundwater, and a growing reliance on groundwater pumping will negatively impact surface water and groundwater connectivity by lowering the water table (
Figure 5). When used simultaneously in a region where baseflow is a crucial component of sustaining Rio Grande streamflow [
56] and traditional acequia irrigation related recharge serves as delayed return flow [
22], the reduction in groundwater recharge and the increase in groundwater pumping will negatively impact surface water availability for downstream users and begin propagating a cycle of maladaptation. It will be critical to prioritize traditional flood irrigation approaches and benefits such as groundwater recharge as much as possible when acequia communities or similar community-based irrigation systems are seeking solutions under conditions of reduced surface water availability.
It is important to distinguish between modernization of irrigation
infrastructure and modernization of irrigation
management. While lining ditches, switching to drip, and supplemental groundwater pumping focus on using surface water more efficiently through engineering and infrastructure improvements, water managers and irrigators must be provided with tools, resources, and information that enable efficient and adaptive water management and allocation. One example of this is real-time monitoring accessible through a web interface which has been shown to increase adaptive capacity indicators within the Rio Hondo acequia community [
55]. With water scarcity only becoming a more pressing issue in the Southwest within the context of climate change, it will be critical to continue evaluating the adaptability of water management and agricultural production approaches, reflect findings in new and transformative policy, and ask ourselves if we should be modernizing infrastructure or management to avoid falling into the irrigation efficiency paradox trap [
78,
79].
A key element for the success of acequia and other community-based irrigation systems is community water management system functionality (see
Figure 5). To have a functioning community water management system, there must first be a community to manage and use the water, so individuals must see value in acequias or acequia irrigation. When researchers explored capital gained within acequia communities, they found that only about 30% of family income was generated from acequia agriculture and that external income helped sustain acequia-irrigated properties and agriculture [
80]. Surveys and interviews revealed that connection to land, water, and community were the values that drove acequia community members to respond to adverse circumstances (e.g., economic hardship, population growth, drought, increased development), demonstrating that acequia communities are founded and fueled by values within the
moral economy rather than the typical market economy [
30,
59,
80]. Therefore, identifying appropriate irrigation modernization recommendations must consider irrigation community motivations or values and be tailored toward enabling water management system functionality. While acequias foster long-term resilience, short-term vulnerabilities that impact acequia irrigation are surface water shortages. More work is needed to assess the specific impacts of changing irrigation regimes and technologies in acequia regions and identify adaptations that optimize groundwater recharge while also taking declining surface water availability into consideration.