Glyphosate-resistant horseweed, or mare’s tail , was reported in 2005 and is one of the dominant weeds in and around raisin and tree fruit production areas of the San Joaquin Valley, as well as on roadsides and canal banks in the region . The level of glyphosate resistance in horse weed is relatively low, and resistant plants are usually injured to some degree following glyphosate applications, which suggests that resistance is not due to an altered target enzyme. Genetic comparisons of horseweed accessions from around the state suggest that there have been multiple, independent origins of resistance in this species, rather than the spread of resistance from a single-source population . Hairy fleabane populations resistant to glyphosate were reported in 2007 . Glyphosate resistance in hairy fleabane appears to be similar to resistance in horse weed in that selection has occurred in response to similar management strategies in perennial crops and surrounding areas ; multiple origins of resistance are suspected ; and growth stage and environmental conditions affect the level of resistance . The discovery by Moretti, Hanson et al. of hairy fleabane resistant to both glyphosate and paraquat raises questions about whether a common physiological mechanism is helping to confer resistance to these dissimilar herbicides, and research is ongoing to elucidate these factors. Junglerice resistant to glyphosate was first identified in 2008 in a Roundup Ready corn field in the Sacramento Valley ; since then, glyphosate-resistant junglerice has become widespread in orchards and field crops throughout California . Resistance appears to be due to mutations in the EPSPS target site , although some populations also appear to have enhanced EPSPS activity . Target-site mutations appear to be the most frequent mechanism among the accessions so far collected in California; however,blueberry pot size additional research is ongoing to determine whether the same is true with populations selected in orchards and in other regions of the Central Valley.

Several other common weeds in orchards and vineyards, including Palmer amaranth , three spike goose grass and witch grass , are suspected to have evolved resistance to glyphosate; preliminary research trials by UC researchers and California State University, Fresno, collaborators have been initiated to verify and characterize the putative resistant populations. Since the discovery of herbicide resistant weed bio-types in California, UC research and Cooperative Extension personnel, as well as university and non-university cooperators and students, have conducted locally relevant weed management research in the state. Research and extension efforts have included alternative chemical management techniques using various post emergence and pre-emergence herbicides along with mechanical control measures in an integrated approach. Applied research integrating agronomy, weed control, spray application technology and water management have been useful to regulatory agencies and have had positive impacts on water and air quality, wildlife habitat and water use . Information on the underlying mechanisms and genetic basis of resistance provides useful information to California weed managers in the longer term. This information is broadly applicable to the biology, physiology, evolution and control of weeds in other crops and regions at the local, national and international level. Although this paper has focused on the efforts of UC weed scientists and collaborators, the basic and applied scientific information developed in California supports similar research being conducted in other regions of the country and world. Like many other areas encompassed by the Endemic and Invasive Pests and Diseases Strategic Initiative, solutions to herbicide resistance are not simple and are affected by many biological, economic, regulatory and social factors. The diverse network of weed scientists and collaborators in a land-grant university system is well positioned to address these complex issues and respond to stakeholder concerns through applied and basic research, extension and outreach to affected agricultural industries, and education of future scientists and leaders. Ultimately, the goal of weed science research is to help growers maintain agricultural productivity and economic stability while increasing environmental sustainability.

Increasing groundwater use for agriculture and public utilities in the last century have put pressure on and diminished groundwater storage in California’s aquifers. The severe droughts that occurred over the last decade were exceptionally warm and dry, including some of the driest years since the late nineteenth century, further exacerbating the adverse effects of decreased ground water water resources . Years of decreased precipitation and increased groundwater extraction have rendered many of California’s groundwater basins and sub-basins to be in a state of groundwater overdraft, where out fluxes of groundwater through pumping or other natural processes greatly exceed influxes to groundwater storage. State agencies recently tasked with achieving groundwater sustainability by 2040, known as Groundwater Sustainability Agencies , have taken action to correct how we can locally manage groundwater resources in California in order to combat groundwater overdraft through the Sustainable Groundwater Management Act . Given how crucial groundwater is to California’s growing population and massive agricultural industry, it is imperative to practice sustainable management of this vital resource. The consequences of mismanagement of California’s groundwater resources are the driving force behind the implementation of methods that can help restore and increase groundwater storage in aquifer systems across the Central Valley. Rising in prevalence as a way of both maintaining and improving groundwater levels is managed aquifer recharge , a process that intentionally places more water into groundwater aquifers than would naturally occur using surface spreading or injection methods . The method of MAR used in this project, agricultural-managed aquifer recharge , spreads diverted surface water onto fallow agricultural fields to recharge groundwater supplies and store water for future use. The feasibility of MAR in agricultural settings depends on water availability, infrastructure, crop tolerance, and the suitability of soil to allow for deep percolation .

The suitability of soil in agricultural fields can be assessed using the Soil and Agricultural Groundwater Banking Index to determine if ag-MAR would be a viable and successful method of replenishing groundwater in a certain area, depending on the rate of deep percolation through the material, residence time of water in the root zone, soil chemistry, as well as topographic and other surface conditions . As long as conditions are viable, ag-MAR can be implemented annually, providing a reliable and sustainable source of stored groundwater to be used in times of drought when other water sources are deficient. In the coming years, GSAs may look towards ag-MAR as a powerful tool in sustainable groundwater management. However, ag-MAR is not only a tool to replenish our groundwater resources, as this application has multiple benefits to an environmental system,raspberry container size including nearby communities and wildlife. There are a limited number of studies that assess the multiple benefits of ag-MAR projects in addition to the apparent hydrologic response in a field’s underlying water table. Although ecological benefits may be harder to measure than hydrologic benefits, considering benefits to wildlife and the environment as design outcomes may have a positive influence on gaining stakeholders to implement more ag-MAR projects. MAR projects undoubtedly have the potential to bridge the gap between two distinct but connected fields of science. Ag-MAR projects are important, especially in California’s Central Valley, because they provide a sustainable way to manage and store groundwater while also being an ecological asset to migratory birds and other organisms that depend on wetland habitats . Across California, groundwater extraction accounts for 40% of the water supply for farms and cities . Colusa County, like many counties in the Central Valley, is highly dependent on pumping groundwater to support their agricultural production. Increased groundwater pumping has resulted in groundwater level declines of >20 ft over the last decade, which highlights the necessity of practicing sustainable groundwater management at the local level. Implementing ag-MAR in Colusa County gives support to farmers while recharging groundwater resources for local communities’ future water usage. The Nature Conservancy , in partnership with Colusa Groundwater Authority, has developed a multi-benefit recharge program to compensate farmers that volunteer their fields to be flooded during the fall season when water resources are limited but migratory bird and waterfowl habitat are in high demand during the fall migratory season . Desirable conditions in the fields that are conducive to both recharge and bird stopovers are those that best mimic a natural wetland habitat. The idea of the TNC project is to convert agricultural fields to temporary wetland habitats that have enough standing water and are free of orchards and other trees that would limit space in the fields and inhibit the bird’s eye view of the fields from above, as migrating birds are more inclined to stop in open flooded fields . Incentive programs like TNC’s on-farm multi-benefit recharge program or BirdReturns, are known to produce a large proportion of open water habitats in post-harvest rice fields during times of drought . BirdReturns and other incentive programs were responsible for providing, on average, 35% of the wetland habitat on the landscape during the 2013-2015 drought, with a few days even reaching up to 100% of the wetland habitat . Previous results of TNC’s incentive program have shown some of the largest average densities in shorebird presence in this agricultural region when wetland habitat was provided for migratory birds that are usually unable to stop in fallow rice fields . The timing of flooding during the fall also makes these ag-MAR sites valuable habitats for birds during migration season when habitats are in deficit, especially during drought .

Just as incentive programs provide a means of sustaining migratory bird populations during dry years, they also provide a way of restoring groundwater resources for use during drought, which further highlights the importance of multi-benefit recharge programs.The goal for this study was to develop two groundwater models using MODFLOW in order to simulate and understand the effects of conducting ag-MAR on selected field sites in Colusa County during the fall season. The first model developed was a large-scale regional model, called the parent model, which was built to derive a more refined, and local child model, which mainly focused on selected recharge sites. The development of the parent and child models allowed us to quantitatively and qualitatively assess the benefits of ag-MAR on the study area’s groundwater resources, the water supply of nearby communities, and as a wetland habitat for migratory birds. In addition to quantifying the effects to these beneficial users, with the results of our models we aimed to answer the following key research questions:1. Through the process of groundwater model development and analysis of results, what guidelines can we provide for optimizing the design of multi-benefit groundwater recharge projects like this in the future? 2. How does the timing, frequency, and amount of recharge affect the results of our model, and what are the benefits of changing each factor? Also, what would we need to monitor to measure these benefits? 3. In what ways can we use groundwater models in the context of understanding hydrologic and environmental impacts in multi-benefit recharge projects, and what answers can we derive from such models?The project area is located in Colusa County, California, in the northern Sacramento Valley region . Located in the Colusa groundwater subbasin of the Sacramento Valley groundwater basin, the project area is bounded by the foothills of the Coast Ranges in the west, and the Sacramento River and other surface water features in the east near the Sutter Buttes. The topography of the project area is mostly flat agricultural land, with higher topography in the southwestern area near the foothills of the Coast Ranges and in the east near the Sutter Buttes. With the exception of a few major urban centers and wildlife refuges, the land use in the area is predominantly agricultural. Agriculture in the area is supported with irrigation water supplied to growers via surface water features like canals and supply systems .Major surface water features in the study area include the Sacramento River, Butte Creek, Butte Slough, the West Borrow Ditch, the Colusa Trough, and the Glenn-Colusa Canal. The Sacramento River flows north to south along the eastern border of the parent model domain and serves as the principal stream in the Colusa Subbasin, significantly contributing to California’s water supply . Regionally, streams that drain the Coast Ranges and Sierra Nevada serve as tributaries to the Sacramento River .