Water management strategies need to balance water security and food production, particularly in semi-arid regions wherein irrigation is required to supplement rainfall. Irrigated stream–aquifer systems present a unique challenge in this effort, due to complex groundwater–surface water interactions and the high level of human intervention in managing irrigation practices. This paper has two objectives: first, to detail a method for constructing and applying a coupled SWAT-MODFLOW to irrigated stream–aquifer systems; and second, to use the model to quantify the effects of decreasing irrigation on hydrological responses and crop yield. The method is applied to a 734 km² study region in the Lower Arkansas River Valley, an alluvial valley in Colorado, USA, which has been intensively irrigated for over 100 years and is threatened by shallow water tables. Therefore, a reduction in applied irrigation amounts has the double benefit of conserving water and decreasing waterlogging, given that crop yield can be maintained for food production. The results indicate that an approximate 10% decrease in total applied irrigation water results in decreases of 6% in surface runoff, 8% in evapotranspiration, and 4% in recharge water. It also results in an increase of 4% in groundwater return flow to the Arkansas River, and an actual increase in groundwater levels due to the decrease in groundwater pumping, pointing to the need for targeted irrigation reduction strategies to decrease waterlogging occurrence. The irrigation reduction yields an average 9% decrease in corn and alfalfa yield. This modeling approach is in general transferable to other similar irrigated river valleys.

Landslides are considered a natural process, with hundreds of events occurring every year in many regions of the world. However, human activities can significantly affect how stable a slope or cliff is, increasing the chances of slope collapses. Moreover, agricultural irrigation has potential to saturate subsurface materials well below ground level and is known to be an important factor that can trigger landslides in many countries. A macroregional literature review of irrigation-induced landslides was developed in this investigation, considering what has been published in Chinese, English, and Spanish. A total of 115 peer-reviewed papers, books and book chapters, graduate and undergraduate theses, and government reports were found, including 82 case studies (23 in Chinese, 26 in English, and 33 in Spanish). Results from this analysis indicate that studies focusing on this important topic have increased exponentially since the 1960s, with most irrigation-induced landslides occurring in dry climates (precipitation less than 600 mm/year), with rainfall concentrated during summer months. The majority of studies have been done in the loess region of China (Asian region), followed by Peru (Latin American region), though cases were found from other macroregions (African, Indian, Russian, Angloamerican, and Indonesian). Based on this global review, new agricultural irrigation projects located in collapsible areas must include a landslide risk analysis. Cultivated areas can follow a series of measures to minimize the chances of triggering a landslide, which would put human lives, ecosystems, food production, and infrastructure at risk.

The Soil and Water Assessment Tool (SWAT) is a well-established modeling tool for simulating hydrologic processes in watershed systems. However, SWAT often does not perform well in highly managed watersheds with intensive irrigation due to management practices and associated hydrologic processes (e.g., canal seepage) not included in the modeling code. This study presents a method to apply SWAT to managed irrigated watersheds, which includes (1) designating each cultivated field as a hydrologic response unit (HRU) and including recorded crop rotations; (2) applying scheduled irrigation according to water rights; and (3) simulating seepage from earthen irrigation canals, with the latter requiring a minor modification to the SWAT code. SWAT’s autoirrigation function is also used as a comparison. The methodology was applied to a $732\text{-}{\mathrm{km}}^2$ watershed in the Lower Arkansas River Valley (southeastern Colorado), a semiarid region that has been intensively irrigated for over 100 years. The model was tested against monthly stream discharge at five stream gauging stations in the Arkansas River and its tributaries during the 2001–2007 period. Results demonstrate the necessity of including human management features, particularly canal seepage, to capture in-stream flows in the river and tributaries. The scheduled irrigation approach has a significantly better performance in the smaller highly agricultural tributaries whereas the autoirrigation functionality has a slightly but not significantly better performance at the outlet of the whole watershed. The methods presented herein can be applied to other semiarid irrigated regions.

Salinity is one of the most common water quality threats in river basins and irrigated regions worldwide. However, no available numerical models simulate all major processes affecting salt ion fate and transport at the watershed scale. This study presents a new salinity module for the SWAT model that simulates the fate and transport of eight major salt ions (SO42-, Ca2+, Mg2+, Na+, K+, Cl−, CO32-, HCO3-) in a watershed system. The module accounts for salt transport in surface runoff, soil percolation, lateral flow, groundwater, and streams, and equilibrium chemistry reactions in soil layers and the aquifer. The module consists of several new subroutines that are imbedded within the SWAT modelling code and one input file containing soil salinity and aquifer salinity data for the watershed. The model is applied to a 732 km2 salinity-impaired irrigated region within the Arkansas River Valley in southeastern Colorado and tested against root zone soil salinity, groundwater salt ion concentration, groundwater salt loadings to the river network, and in-stream salt ion concentration. The model can be a useful tool in simulating baseline salinity transport and investigating salinity best management practices in watersheds of varying spatial scales.

Majes is one of the largest agricultural areas in the Arequipa region (southern Peru). Low seasonal precipitation and increasing water demands for agricultural irrigation, industry, and human consumption have made water supply projections a major concern. Agricultural development is becoming more extensive in this dry, sunny climate where crops can be grown year-round. However, because this type of project usually involves significant perturbations to the regional water cycle, understanding the effects of irrigation on local hydrology is crucial. Based on the watershed-scale Soil and Water Assessment Tool (SWAT), this investigation focuses on the impacts of intensive irrigation on hydrological responses in the Majes region. This study is unique because we allow for crop-field scale input within our regional-scale model to provide information at this smaller scale, which is important to inform local stakeholders and decision makers. Each hydrologic response unit (HRU) was generated to represent an individual crop field, so that management practices could be applied according to real-world scenarios. The management file of each HRU was modified to include different operation schedules for crop rotation, irrigation, harvest, and tillage. The model was calibrated and validated against monthly observed stream discharge during the 2009–2020 period. Additionally, evapotranspiration, irrigation water volume, and daily stream discharge downstream of the local river (Siguas) were used to verify the model performance. A total of 49 sub-basins and 4222 HRUs were created, with 3000 HRUs designated to represent individual crop fields. The simulation results revealed that infiltration from agricultural activities in Majes represents the majority of annual groundwater return flow, which makes a substantial contribution to streamflow downstream of the Siguas River. Simulations also suggested that groundwater flow processes and the interactions between surface and groundwater have a major impact on the water balance of the study area. Additionally, climate variability had a higher impact on surface runoff than on groundwater return flow, illustrating that the groundwater component in the study area is important for future water resources resiliency under expected climate change scenarios. Finally, there is a need to perform a follow-up implementation to provide a guideline for decision-makers to assess future sustainable water resources management under varying climatic conditions for this arid irrigated system.