It is likely that these situations respond differently to CA compared with other regions where CA has been evaluated in less-intensive systems and under rain-fed situations.One unique aspect of South Asia is the small size of farms, and many farmers cannot afford to own equipment. More service providers and promotion through policies, loans and training are needed to accelerate the adoption of CA. Sensible approaches to agricultural development in the South Asia region will embrace the evidence based potential of the practices while being realistic about the magnitude of achievable gains and deploying more targeted approaches to scaling that prioritize crops and soil types where expected benefits are the highest.Information on salinity processes operating within a watershed is critical towards evaluating ecosystem services or ES of the river system. Modification of the water cycle through agriculture can lead to changes in the magnitude and timing of water flows, contributing to shifts in river pollution states in downstream water bodies, soil moisture regimes, and microclimates . These changes can have enormous consequences for long-term food production. In most parts of the world, the regime shifts to which a particular region may be vulnerable under different land uses or under conditions such as a changing climate are largely unknown, as are the effects of potential regime shifts on ecosystems, ecosystem services,macetas plastico and human well-being. Such information is critical to sustainable development planning and to assessments of social and ecological resilience, which are increasingly central to development policy .
Several studies underscore the importance of studying and interpreting patterns of human modification in the landscape to understand deeply the consequences of human intervention in the past and to better plan engineered responses to future challenges . Wagener et al. and Grismer have called for a new paradigm for hydrologic science that includes human-induced changes as integral to the overall hydrologic system. Recently, Pande and Sivapalan , M. Sivapalan et al. , Kandasamy et al. , and Murugesu Sivapalan et al. proposed the sub-field ‘socio-hydrology’ with “a focus on the understanding, interpretation and prediction of the flows and stocks in the human modified water cycle at multiple scales, with explicit inclusion of the two-way feed backs between human and water systems” to address these challenges. Changes in ecosystems and social-ecological systems are usually experienced as relatively slow and incremental, but from time-to-time dramatically large, persistent, and often unexpected changes take place. Such large, persistent changes are commonly referred to as regime shifts . Understanding of how agricultural modifications of the hydrological cycle regulate the prevalence and severity of surprising nonlinear change is lacking . However, a growing body of evidence suggests that agricultural modification of the quantity and quality of hydrological flows can increase the risk of ecological regime shifts . Applying ecosystem resilience and sustainability at an operational level requires understanding the linkages between socioeconomic and natural systems . Using literature, Reinette Biggs et al. identified seven principles for enhancing the resilience of desired ES in the face of disturbance and ongoing change in SES. Their principle 3 outlined a need to manage slow variables and feed backs for maintaining SES regimes that underlie the production of desired ES.
Applying this principle, however, requires understanding of the possible regime shifts and their consequences as well as the key factors that trigger such regime shifts. Here, we examined long-term records of surface water salinity to produce both understanding and perspective toward managing salinity in arid and semi-arid regions while also enabling the study of the influences of external climatic variability and internal drivers in the system. Agriculture is now the largest anthropogenic disturbance in the watershed in terms of area, followed by urbanization and dam emplacement . As well, J. G. Thompson and Reynolds noted the major project goals of the development of the Salinas Valley Basin Management Plan by the Monterey County Water Resources Agency since 1992 was to sustain and even further develop profitable agricultural enterprises. As such, our aim was to quantify the dynamic relationships between solute concentration and stream discharge to improve our understanding of solute transport pathways and active source areas in the Salinas watershed. Elucidation of temporal dependence in solute dynamics of a highly developed, semi-arid basin is a forensic exercise of implicating and eliminating a host of potential contributing factors, or controls. When discrete controls on solute dynamics are discovered in a developed watershed, it is often the result of scenarios where proportionally large areal disturbances have dominated the solute responses; in this way, urbanization and agriculture have been found to exert significant control on stream solute fluxes.The 1972 Clean Water Act is the principal instrument of non-point pollution reform in the USA. This Act mandates ‘listing’ of all impaired water bodies in the USA, and development of Total Maximum Daily Load plans to address the impairment. The lower Salinas River is 303d listed for salinity and nutrients .
Furthermore, the 2015 Senate Bill 390 and later California Superior Court ruling required the Central Coast Water Quality Control Board to develop more comprehensive water quality protections. In 2017, the Regional Board adopted conditional waivers for dischargers from irrigated lands which required monitoring, reporting, completion of annual compliance forms, development of irrigation, nutrient management, and water quality buffer plans depending on one of three applicable ‘Tiers’ of land use. These are stringent requirements that are costly for farmers and to be successful require that farmers understand and trust that their efforts will result in improved water quality overtime . Studies that provide quantitative links between land management practices and water quality outcomes are therefore essential. Solute fluxes in streams from developed watersheds in semi-arid environments are influenced by natural and human-induced changes to the land surface that interact with variable climatic regimes. The causes of saline influxes to rivers can be due to natural processes or to secondary processes arising from anthropogenic changes in the catchment. Secondary salinization arises from a number of factors such as changing land use and rising saline water tables discharging into the rivers and saline soil pools. For example, saline flows in the Colorado River have increased over the last 100 years presumably associated with water diversions and upstream developments. Natural salinization processes in semi-arid zone catchments are generally considered to result from dry and wet deposition of marine salts by rainfall , re-suspended regional dust , and rock weathering . The latter concentrates in the root and unsaturated zones eventually seeping to deeper groundwater or is collected by artificial subsurface drainage systems. The complexity of the processes affecting surface to soil-water systems makes it difficult to unravel the extent of natural salinization from the accelerated processes caused by anthropogenic influences. Long-term monitoring indicates that solute concentration behavior often exhibits temporal dependence over event to inter-decadal time scales particularly in arid to semi-arid climates . Factors affecting watershed-scale solute production operate over a wide range of time scales,cultivar arandanos with even seemingly discrete events generating legacy effects that may endure for years. That is, salinity and nitrate concentrations in the catchment may be responding to land use changes from decades prior. In this context, salinity and nitrate management measures introduced historically cannot be judged successful without an ability to communicate water quality changes, particularly changes in forcing factors over time that then affect watershed scale solute production and transport. In a study of stream flow nitrate concentrations within a small agricultural catchment of southwest England, Burt and Worrall observed changes in the system state over time and concluded that there was non-stationarity in long-time series. They suggested that long-term records are required to understand the time constants of a range of driving processes. Event to inter annual scale concentration behavior in river systems is known to be influenced by antecedent hydrologic conditions, whereby previous hydrologic activity regulates the concentration-discharge relationship . Responses of solute concentrations to fluctuations in stream flow have been observed to vary between sites, to vary for specific species, and to vary for specific storms.
Magnitudes of antecedent storm events, mixing of event water with groundwater, long periods of low flow, variable chemistry of “old” water, and catchment flushing times have all been related to perturbations of long-term hydro-geochemical response of a catchment . Biron et al. studied the effect of antecedent soil-moisture conditions on stream water quality and found that with dry antecedent conditions, there was a general decrease in solute concentrations with time, whereas concentrations remained about the same under wet conditions. Butturini and Sabater and Piñol et al. studied effects of rainfall variability and consequent flushing of soils on stream chemistry and found that changes in stream fluxes are to the first order, dependent upon changes in hydrology. Temporal variability in stream salinity in the developed Salinas Valley watersheds was determined from “grab sampling” datasets of major salinity components and associated discharges and a complete record of daily stream discharge from 1977 to 2013. Although limited in providing an understanding of solute flux behavior during storm events , long-term “grab sampling” datasets with accompanying instantaneous discharge and mean daily stream discharge datasets for the complete record can be used to estimate the actual history of concentrations and fluxes using concentration– discharge relationships. Continuous monitoring of stream water quality is expensive; however, frequent flow monitoring coupled with long-term stream water quality “grab sampling” is widely available especially in the USGS database. We sought to apply methods that can use these available datasets as diagnostic tools regarding the changes in stream solute concentrations taking place in the watershed of interest. To improve water quality, there is great value in developing and using data analysis methods aimed at deriving the greatest possible amount of information from the data that are collected, particularly related to changes in water quality over time. The overarching objective of this paper was to provide understanding of the stream solute-concentration dynamics in the lower Salinas River over inter-decadal time scales. Our aim was to disentangle multiple controls on river salinity at a regional scale, and to infer mechanisms behind fluctuations of solute concentrations in the lower Salinas River over time. Mountainous highlands of the watershed are mostly composed of Mesozoic-aged sedimentary and meta sedimentary rock with some igneous intrusions, while the northern extent of the main stem valley floor is Tertiary and younger alluvial fill . Maximum relief in the basin is ~1900 m and average watershed bounding ridge heights are 750 m to the NE and 1200 m in the SW, with ridge crest height generally decreasing toward the mouth of the Salinas. Land cover in the Salinas watershed largely follows local relief, with steep forested terrain giving way down slope to chaparral/scrub in the wetter western hills and grassland in the drier eastern hills . The Salinas River originates in the south near Santa Margarita in San Luis Obispo County and flows northwest from an elevation of ~274 to ~61 m near Greenfield along US Highway 101 approximately 32 km to the Pacific Ocean at Monterey Bay, discharging north of the City of Marina. The Salinas, from its junction with the Estrella River to the junction with the San Antonio River, flows on a narrow flood plain . Nearby hills to the east that are 213 to 305 m above the flood plain have blunt steep faces and numerous slides; both features suggest erosion at the base of the hills by the Salinas River. North of its junction with the San Antonio River, the Salinas flows through a narrow canyon. Opposite the mouth of San Lorenzo Creek, the Salinas River is on the west side of the valley; farther north opposite the mouth of the Arroyo Seco, it is on the east. Mean annual discharge of the Salinas River ranges from 350 million m3 /year near Chualar and near ~432 million m3 /year near Bradley below Nacimiento dam. Streamflow near Bradley represents environmental and groundwater recharge releases from the Nacimiento and San Antonio reservoirs with most flow occurring during November through March. These major reservoirs, located along the Nacimiento River and the San Antonio River, were completed in 1957 and 1967, respectively.