On the variability of cold region flooding

Cold region hydrological systems exhibit complex interactions with both climate and the cryosphere. Improving knowledge on that complexity is essential to determine drivers of extreme events and to predict changes under altered climate conditions. This is particularly true for cold region flooding where independent shifts in both precipitation and temperature can have significant influence on high flows. This study explores changes in the magnitude and the timing of streamflow in 18 Swedish Sub-Arctic catchments over their full record periods available and a common period (1990-2013). The Mann-Kendall trend test was used to estimate changes in several hydrological signatures (e.g. annual maximum daily flow, mean summer flow, snowmelt onset). Further, trends in the flood frequency were determined by fitting an extreme value type I (Gumbel) distribution to test selected flood percentiles for stationarity using a generalized least squares regression approach.Results highlight shifts from snowmelt-dominated to rainfall-dominated flow regimes with all significant trends (at the 5% significance level) pointing toward (1) lower magnitudes in the spring flood; (2) earlier flood occurrence; (3) earlier snowmelt onset; and (4) decreasing mean summer flows. Decreasing trends in flood magnitude and mean summer flows suggest widespread permafrost thawing and are supported by increasing trends in annual minimum daily flows. Trends in selected flood percentiles showed an increase in extreme events over the full periods of record (significant for only four catchments), while trends were variable over the common period of data among the catchments. An uncertainty analysis emphasizes that the observed trends are highly sensitive to the period of record considered. As such, no clear overall regional hydrological response pattern could be determined suggesting that catchment response to regionally consistent changes in climatic drivers is strongly influenced by their physical characteristics

Soil suitability index identifies potential areas for groundwater banking on agricultural lands

Groundwater pumping chronically exceeds natural recharge in many agricultural regions in California. A common method of recharging groundwater — when surface water is available — is to deliberately flood an open area, allowing water to percolate into an aquifer. However, open land suitable for this type of recharge is scarce. Flooding agricultural land during fallow or dormant periods has the potential to increase groundwater recharge substantially, but this approach has not been well studied. Using data on soils, topography and crop type, we developed a spatially explicit index of the suitability for groundwater recharge of land in all agricultural regions in California. We identified 3.6 million acres of agricultural land statewide as having Excellent or Good potential for groundwater recharge. The index provides preliminary guidance about the locations where groundwater recharge on agricultural land is likely to be feasible. A variety of institutional, infrastructure and other issues must also be addressed before this practice can be implemented widely

Improved Understanding Of Subsurface Hydrology In Variable Source Areas And Its Implications For Water Quality

Variable source areas (VSAs) are hot spots of hydrological (saturation-excess runoff) and biogeochemical processes (e.g. nitrogen, phosphorus, organic carbon cycling) in the landscapes of the northeastern U.S. Despite the substantial research conducted in the past 50 years, there is still process understanding to be gained on how VSA connect with the surrounding area, how this interaction influences surface and subsurface runoff generation and chemical transport and how these processes can be captured in ungaged basins using watershed models. To determine the controls on VSA formation and connectivity, a 0.5 ha hillslope was instrumented (trenched) in the southern tier of New York, U.S. Water flux from different soil layers in the trench and upslope water table dynamics were recorded for 16 events and isotopic and geochemical tracers were measured during five events. In conjunction with the surface and bedrock topography these measurements allowed detailed characterization of the subsurface storm flow response within the VSA. Analysis revealed that the most important control on storm flow response was antecedent moisture. During events with dry antecedent conditions subsurface flow was dominated by percolation through the fragipan (i.e. cracks and macropores). Flow from below the fragipan showed a constant flow rate (0.8 mm/h), which was independent of storm size and antecedent moisture. Under wet antecedent conditions hydrological connectivity increased and subsurface flow is dominated by lateral flow through the soil atop the fragipan. During these events flow contributing slope length to the trench was five to tenfold increased. Thus, pollutant and nutrient transport from a greater distance has to be considered in water management during events with wet antecedent conditions. Application of the empirical Soil Conservation Service Curve Number method showed that discharge volumes were generally well predicted but revealed that for continuous predictions of VSA dynamics more conceptually coherent solutions need to be developed that consider the effect of antecedent moisture on runoff generation. This research shows that indirect indicators such as the average water table depth, the base flow rate prior to events or water balance estimates of the soil water content can be incorporated into watershed models to improve predictions

Real-Time Forecast of Hydrologically Sensitive Areas in the Salmon Creek Watershed, New York State, Using an Online Prediction Tool

In the northeastern United States (U.S.), watersheds and ecosystems are impacted by nonpoint source pollution (NPS) from agricultural activity. Where agricultural fields coincide with runoff-producing areas—so called hydrologically sensitive areas (HSA)—there is a potential risk of NPS contaminant transport to streams during rainfall events. Although improvements have been made, water management practices implemented to reduce NPS pollution generally do not account for the highly variable, spatiotemporal dynamics of HSAs and the associated dynamics in NPS pollution risks. This paper presents a prototype for a web-based HSA prediction tool developed for the Salmon Creek watershed in upstate New York to assist producers and planners in quickly identifying areas at high risk of generating storm runoff. These predictions can be used to prioritize potentially polluting activities to parts of the landscape with low risks of generating storm runoff. The tool uses real-time measured data and 24–48 h weather forecasts so that locations and the timing of storm runoff generation are accurately predicted based on present-day and future moisture conditions. Analysis of HSA predictions in Salmon Creek show that 71% of the largest storm events between 2006 and 2009 were correctly predicted based on 48 h forecasted weather data. Real-time forecast of HSAs represents an important paradigm shift for the management of NPS in the northeastern U.S

Streamflow availability ratings identify surface water sources for groundwater recharge in the Central Valley

In California's semi-arid climate, replenishment of groundwater aquifers relies on precipitation and runoff during the winter season. However, climate projections suggest more frequent droughts and fewer years with above-normal precipitation, which may increase demand on groundwater resources and the need to recharge groundwater basins. Using historical daily streamflow data, we developed a spatial index and rating system of high-magnitude streamflow availability for groundwater recharge, STARR, in the Central Valley. We found that watersheds with excellent and good availability of excess surface water are primarily in the Sacramento River Basin and northern San Joaquin Valley. STARR is available as a web tool and can guide water managers on where and when excess surface water is available and, with other web tools, help sustainable groundwater agencies develop plans to balance water demand and aquifer recharge. However, infrastructure is needed to transport the water, and also changes to the current legal restrictions on use of such water