Watershed Variables Research Articles

Modeling hillslope sediment yield using rainfall simulator field experiments and partial least squares regression: Cahaba River watershed, Alabama (USA)

Hillslope erosion processes such as rainsplash erosion and slopewash are a major source of fine sediment that causes water-quality problems in many river systems. Hillslope sediment yield is controlled by both climatic and watershed characteristics. Understanding the relative sensitivity of sediment yield to soil and watershed characteristics under different precipitation intensities is important for identifying hotspots of sediment yield and the likely response of sediment yield to changing climate and land use. Here, field experiments on sediment yield were conducted in Alabama’s (United States) Cahaba River watershed under varying precipitation intensities using a rainfall simulator. Using data on soil and watershed characteristics, partial least squares (PLS) regression models were developed of sediment yield under both “more intense” (recurrence interval ≈ 10 years) and “less intense” (recurrence interval ≈ 1 year) simulated rainfall events. The resulting models were used to create spatially explicit estimates of sediment yield under different precipitation intensities for the Cahaba River watershed. The optimized PLS models had a cumulative R 2 of 0.41 and 0.73 for the “more intense” and “less intense” rainfall events, respectively. The higher R2 of the “less intense” model is attributed to higher significance of the heterogeneity of watershed variables under less intense precipitation. Significant variables that were retained in both models included percent sand, percent clay, percent organic matter, and slope. The results support the use of PLS modeling to analyze the results of field experiments using rainfall simulators to examine both climatic and watershed controls on hillslope sediment yield.

Synthesis of Common Management Concerns Associated with Dam Removal

AbstractManagers make decisions regarding if and how to remove dams in spite of uncertainty surrounding physical and ecological responses, and stakeholders often raise concerns about certain negative effects, regardless of whether these concerns are warranted at a particular site. We used a dam‐removal science database supplemented with other information sources to explore seven frequently raised concerns, herein Common Management Concerns (CMCs). We investigate the occurrence of these concerns and the contributing biophysical controls. The CMCs addressed are the following: degree and rate of reservoir sediment erosion, excessive channel incision upstream of reservoirs, downstream sediment aggradation, elevated downstream turbidity, drawdown impacts on local water infrastructure, colonization of reservoir sediments by nonnative plants, and expansion of invasive fish. Biophysical controls emerged for some of the concerns, providing managers with information to assess whether a given concern is likely to occur at a site. To fully assess CMC risk, managers should concurrently evaluate site conditions and identify the ecosystem or human uses that will be negatively affected if the biophysical phenomenon producing the CMC occurs. We show how many CMCs have one or more controls in common, facilitating the identification of multiple risks at a site, and demonstrate why CMC risks should be considered in the context of other factors such as natural watershed variability and disturbance history.

Statistically Extracted Fundamental Watershed Variables for Estimating the Loads of Total Nitrogen in Small Streams

Accurate estimation of total nitrogen loads is essential for evaluating conditions in the aquatic environment. Extrapolation of estimates beyond measured streams will greatly expand our understanding of total nitrogen loading to streams. Recursive partitioning and random forest regression were used to assess 85 geospatial, environmental, and watershed variables across 636 small (<585 km2) watersheds to determine which variables are fundamentally important to the estimation of annual loads of total nitrogen. Initial analysis led to the splitting of watersheds into three groups based on predominant land use (agricultural, developed, and undeveloped). Nitrogen application, agricultural and developed land area, and impervious or developed land in the 100-m stream buffer were commonly extracted variables by both recursive partitioning and random forest regression. A series of multiple linear regression equations utilizing the extracted variables were created and applied to the watersheds. As few as three variables explained as much as 76 % of the variability in total nitrogen loads for watersheds with predominantly agricultural land use. Catchment-scale national maps were generated to visualize the total nitrogen loads and yields across the USA. The estimates provided by these models can inform water managers and help identify areas where more in-depth monitoring may be beneficial.

Evaluating Key Watershed Components of Low Flow Regimes in New England Streams.

Water resource managers seeking to optimize stream ecosystem services and abstractions of water from watersheds need an understanding of the importance of land use, physical and climatic characteristics, and hydrography on different low flow components of stream hydrographs. Within 33 USGS gaged watersheds of southern New England, we assessed relationships between watershed variables and a set of low flow parameters by using an information-theoretical approach. The key variables identified by the Akaike Information Criteria (AIC) weighting factors as generating positive relationships with low flow events included percent stratified drift, mean elevation, drainage area, and mean August precipitation. The extent of wetlands in the watershed was negatively related to low flow magnitudes. Of the various land use variables, the percentage of developed land was found to have the highest importance and a negative relationship on low flow magnitudes, but was less important than wetlands and physical and climatic features. Our results suggest that management practices aimed to sustain low flows in fluvial systems can benefit from attention to specific watershed features. We draw attention to the finding that streams located in watersheds with high proportions of wetlands may require more stringent approaches to withdrawals to sustain fluvial ecosystems during drought periods, particularly in watersheds with extensive development and limited deposits of stratified drift.

Hydrological variability of the Soummam watershed (Northeastern Algeria) and the possible links to climate fluctuations

The long-term variability of rainfall in the Soummam watershed (NE Algeria) has been analysed over the past 108 years using continuous wavelet method in order to identify the interannual modes controlling the rainfall variability. Statistical analyses of rainfall timeseries have shown its distribution following five periods of time, limited by a series of discontinuities around 1935, 1950, 1970 and 1990. The continuous wavelet transform have demonstrated different low frequency modes: 2–4, 4–8, 8–16 and 16–32 years.

Identifying Watershed, Landscape, and Engineering Design Factors that Influence the Biotic Condition of Restored Streams

Restored stream reaches at 79 sites across North Carolina were sampled for aquatic macroinvertebrates using a rapid bioassessment protocol. Morphological design parameters and geographic factors, including watershed and landscape parameters (e.g., valley slope, substrate), were also compiled for these streams. Principal component regression analyses revealed correlations between design and landscape variables with macroinvertebrate metrics. The correlations were strengthened by adding watershed variables. Ridge regression was used to find the best-fit model for predicting dominant taxa from the “pollution sensitive” orders of Ephemeroptera (mayflies), Plecoptera (stoneflies), and Trichoptera (caddisflies), or EPT taxa, resulting in coefficient weights that were most interpretable relative to site selection and design parameters. Results indicate that larger (wider) streams located in the mountains and foothills where there are steeper valleys, larger substrate, and undeveloped watersheds are expected to have higher numbers of dominant EPT taxa. In addition, EPT taxa numbers are positively correlated with accessible floodplain width and negatively correlated with width-to-depth ratio and sinuosity. This study indicates that both site selection and design should be carefully considered in order to maximize the resulting biotic condition and associated potential ecological uplift of the stream.

Geospatial-based soil variability and hydrological zones of Abha semi-arid mountainous watershed, Saudi Arabia

Soil and water resources are vital to a society because these resources maintain the environment and living being. Understanding the inconsistencies of these resources has significant importance for regional planning and landscape management. Though their relevance, there is no updated data and organized method for characterization and mapping of soil database and hydrological system in Abha watershed area. The objective of this study is to develop a soil geodatabase and generation of hydrological zones. The methodology is based on the Geoinformatics techniques to determine the causative factors that affect the hydrology and to delineate hydrological zones. Ten parameters were considered to delineate the hydrological zones based on the literature review and thorough discussion with international scientific experts (i.e., engineers, hydrologists, and hydrogeologists). The themes and their classes were assigned suitable weights on Saaty’s scale according to their relative importance. The assigned weights of the themes and their classes were normalized by analytic hierarchy process and eigenvector method. Thereafter, the all themes were integrated in geographical information system (GIS) using weighted linear combination method to create the hydrological map. Thus, five hydrological zones were identified and demarcated in the study area, viz. “very low runoff,” “low runoff,” “moderate runoff,” “high runoff,” and “very high runoff” based on surface runoff potential index values. This analysis also shows that 5.09 and 32.29 % of the watershed lies in the very high runoff and high runoff zone, followed by moderate and low runoff zone with 28.17 and 25.22 % areal coverage, respectively. With this qualitative-based classification, the high runoff generation area of the Abha watershed has a large area coverage, indicating the availability of high surface water potential. Additionally, the results showed that the runoff distribution in the western, southwestern, and central north highlands of the watershed is high and these areas flow to the down wadies. Thus, the contribution of this work in understanding the Abha watershed hydrologic system is significant. The results provide valuable information of Abha watershed about the hydrological zone and soil database at a large scale for the first time. Generated maps will assist to formulate effective runoff utilization plans so as to ensure long-term sustainability.

Adaptive Management in Water Law: Evaluating Australian (New South Wales) and Canadian (British Columbia) Law Reform Initiatives

Sub-national jurisdictions are undertaking law reform that attempts to remedy the historic inflexibility in water law and to provide security in water use entitlements. These reforms respond to increasing hydrological variability in many watersheds where the volume of water available for consumption at the times of highest demand is decreasing and minimum environmental flows are a precursor to healthy ecological systems. Adaptive management is a foundational ecological and ecosystem-based management principle. Scholars and professionals from a myriad of disciplines are calling for legal and policy structures that allow adaptive environmental management regimes based on evolving watershed conditions. Adaptive management in a water context requires integrated decision- making that incorporates land use decisions with decisions about surface and groundwater, provision for minimum environmental flows, and the ability to alter water users’ entitlements when a water resource is either over-allocated or changing flow regimes results in insufficient water. The purpose of this paper is to compare the water law reforms of the state of New South Wales in Australia and the province of British Columbia in Canada as they attempt to incorporate legal and management tools for adaptive management and to address fixed entitlements for water use in favour of more responsive and watershed-specific management approaches. In New South Wales, adaptation in water law is based on a watershed plan and allocation of a fluctuating consumptive pool of water, which is the amount of water available for extraction under licence after environmental needs are taken into account. In British Columbia, the provincial government may adapt water licences over time through water sustainability planning and water licence review, as well as issue short-term orders restricting water diversions without needing to compensate licence holders for any damages owing from these orders. These reforms are leading the restructuring of water law to address conflicts between water users as well as between water users and the environment. These reforms are also at the forefront internationally as models for adaptive management. They provide some flexibility in accommodating environmental flows but decrease certainty for water users as hydrological systems change. Their relative successes in accounting for adaptation and healthy hydrological systems will be instructive to other jurisdictions as they move to align their water laws with principles of adaptive management.

Multi-scale assessment of habitats and stressors influencing stream fish assemblages in the Lake Pontchartrain Basin, USA

This study determined if a spatial hierarchy existed with respect to reach-scale habitat, subwatershed-scale, and watershed-scale geomorphology and land use stressors on fish assemblages in southeastern U.S. coastal plain streams. During May–October 2009–2012, fishes were sampled by seine, and habitat was assessed at 50 reaches in the Lake Pontchartrain Basin (USA). Using partial redundancy analysis (pRDA) a variance decomposition procedure was used to partial out influences of confounding covariables at each spatial scale. Reach-scale habitat had the strongest association with the assemblage. Stream width, depth, aquatic vegetation and human debris cover, rapid habitat assessment score, and large woody debris volume were the most important variables. At subwatershed and watershed scales, natural and anthropogenic characteristics were important, including elevation, gradient, watershed area, wetland cover, stream density, road, dam and oil/gas well densities. Six species were associated most strongly with the watershed variables, compared to reach- and subwatershed-scale variables. These species had more “r-selected” life-history strategies (e.g., smaller eggs, shorter life spans, multiple broods, longer spawning season, and trophic generalists). In contrast, most species that were associated strongest with reach-scale variables exhibited more “k-selected” life-history traits (e.g., larger eggs, longer life spans, shorter spawning season, single brood, and trophic specialists).

Habitat Restoration in the Context of Watershed Prioritization: The Ecological Performance of Urban Stream Restoration Projects in Portland, Oregon

AbstractIn Portland (Oregon, USA), restoration actions have been undertaken at the watershed scale (e.g. revegetation and stormwater management) to improve water quality and, where water quality and quantity are adequate at the reach scale, to increase habitat heterogeneity. Habitat enhancement in urban streams can be important for threatened species, but challenging, because of altered catchment hydrology and urban encroachment on floodplains and channel banks. To evaluate reach‐scale restoration projects in the Tryon Creek watershed, we sampled benthic macroinvertebrates and conducted habitat quality surveys pre‐project and over 4 years post‐project. Species sensitive to pollution and diversity of trophic groups increased after restoration. Taxonomic diversity increased after restoration but was still low compared with reference streams. We found no significant changes in trait proportions and functional diversity. Functional diversity, proportion of shredders and semivoltine invertebrates were significantly higher in reference streams than in the restored stream reaches. We hypothesized that inputs of coarse particulate organic matter and land use at watershed scale may explain the differences in biodiversity between restored and reference stream reaches. Habitat variables did not change from pre‐project to post‐project, so they could not explain community changes. This may have been partly attributable to insensitivity of the visual estimate methods used but likely also reflects the importance of watershed variables on aquatic biota—suggesting watershed actions may be more effective for the ecological recovery of streams. For future projects, we recommend multihabitat benthic sampling supported by studies of channel geomorphology to better understand stream response to restoration actions. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Paleoenvironmental Studies in Southwestern Yukon

The St. Elias Mountain region has occupied an important place in the study of the Quaternary because it presents a relatively accessible non-polar icefield and an array of environments from tundra to boreal forest. Paleoenvironmental studies in southwestern Yukon have documented the broad-scale climatic changes of the past 20 000 years, although few studies exist with well-dated sequences at high temporal resolution. Picea glauca arrived across the entire region around 10 000 years ago; however, the details regarding its migration pathways are not well known. Available records indicate few major changes in the composition of the boreal forest vegetation since that time. A slightly more intense fire regime in the early to mid Holocene has been suggested, but this conclusion is based on only a few studies. Variations in the tree line during the Holocene have been examined, but these studies also lack details. There is no evidence for more extensive grasslands in the area during the Holocene. Paleolimnological studies indicate that changes in populations of aquatic organisms have occurred in response to either Holocene climates or watershed variability.

Effect of Native Vegetation Loss on Stream Ecosystem Processes: Dissolved Organic Matter Composition and Export in Agricultural Landscapes

Stream and river ecosystems are dependent on energetic inputs from their watersheds and thus shifts in land use from forest cover to agriculture will affect stream community composition and function. The disruption of forest-aquatic linkages alters the organic matter resources in agricultural streams. Dissolved organic matter (DOM) is the dominant form of organic matter in aquatic ecosystems, and a microbial energy source that is important for stream respiration. The concentrations and characteristics of DOM are regulated by both terrestrial (for example, terrestrial organic matter supply) and in-stream processes (for example, microbial respiration and periphyton production) that are influenced by land management. The effects of watershed land use and topographic, soil and climatic variables on DOM quantity (dissolved organic carbon concentration and load), source (terrestrial or in-stream) and quality (composition and lability) were measured in 14 streams across an agricultural land-use gradient. DOC concentration was positively correlated with watershed pasture cover and negatively correlated with watershed relief. No watershed variables were important correlates of DOC load. Stream DOM was primarily of terrestrial origin, but DOM in agricultural streams had a greater proportion of sources from in-stream sources. This may be due to reduced connection with riparian vegetation and increased in-stream primary production. We suggest that maintaining watershed tree cover greater than 52% and ensuring less than 10% of the length of riparian corridor is cleared for pasture could minimize changes to DOM composition. This is important to avoid flow-on effects for stream ecosystem processes that are mediated by DOM. Long-term DOM monitoring will be valuable for assessing the functional impacts of land-use change.

Local‐scale and watershed‐scale determinants of summertime urban stream temperatures

AbstractThe influence of urbanization on the temperature of small streams is widely recognized, but these effects are confounded by the great natural variety of their contributing watersheds. To evaluate the relative importance of local‐scale and watershed‐scale factors on summer temperatures in urban streams, hundreds of near‐instantaneous temperature measurements throughout the central Puget Lowland, western Washington State, were collected during a single 2‐h period in August in each of the years 1998–2001. Stream temperatures ranged from 8.9 to 27.5 °C, averaging 15.4 °C. Pairwise correlation coefficients between stream temperature and four watershed variables (total watershed area and the watershed percentages of urban development, upstream lakes, and permeable glacial outwash soils as an indicator of groundwater exchange) were uniformly very low. Akaike's information criterion was applied to determine the best‐supported sets of watershed‐scale predictor variables for explaining the variability of stream temperatures. For the full four‐year dataset, the only well‐supported model was the global model (using all watershed variables); for the most voluminous single‐year (1999) data, Akaike's information criterion showed greatest support for per cent outwash (Akaike weight of 0.44), followed closely by per cent urban development + per cent outwash, per cent lake area only, and the global model. Upstream lakes resulted in downstream warming of up to 3 °C; variability in riparian shading imposed a similar temperature range. Watershed urbanization itself is not the most important determining factor for summer temperatures in this region; even the long‐recognized effects of riparian shading can be no more influential than those imposed by other local‐scale and watershed‐scale factors. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Lakes without Landsat? An alternative approach to remote lake monitoring with MODIS 250 m imagery

We evaluated use of MODIS 250 m imagery for remote lake monitoring in Maine. Despite limited spectral resolution (visible red and near infrared bands), the twice daily image capture has a potential advantage over conventionally used, often cloudy Landsat imagery (16 day interval) when short time windows are of interest. We analyzed 364 eligible (≥100 ha) Maine lakes during late summer (Aug–early Sep) 2000–2011. The red band was strongly correlated with natural log-transformed Secchi depth (SD), and the addition of ancillary lake and watershed variables explained some variability in ln(SD) (R2 = 0.68–0.85; 9 models). Weak spectral resolution and variable lake conditions limited accurate lake monitoring to relatively productive periods in late summer, as indicated by inconsistent, sometimes weak regressions during June and July when lakes were clearer and less stable (R2 = 0.19–0.74; 8 models). Additionally, SD estimates derived from 2 sets of concurrent MODIS and Landsat imagery generally did not agree unless Landsat imagery (30 m) was resampled to 250 m, likely owing to various factors related to scale. Average MODIS estimates exceeded those of Landsat by 0.35 and 0.49 m on the 2 dates. Overall, MODIS 250 m imagery are potentially useful for remote lake monitoring during productive periods when Landsat data are unavailable; however, analyses must occur when algal communities are stable and well-developed, are biased toward large lakes, may overestimate SD, and accuracy may be unreliable without non-spectral lake predictors.

Is a trout a trout? A range-wide comparison shows nonnative brook trout exhibit greater density, biomass, and production than native inland cutthroat trout

Frequently nonnative species invade habitats occupied by a similar, even closely related, species, and it is often assumed that the two have functionally redundant ecological roles. However, the consequences of these types of invasions are rarely studied. Eastern brook trout are the most widely distributed and abundant nonnative fish throughout western North America and are assumed to be analogous in terms of ecological function to the native cutthroat trout they replace. However, based on results of previous studies conducted on a few streams, we hypothesized that the two species differ ecologically, and we predicted that brook trout would exhibit greater density, biomass, and annual production than cutthroat trout at the onset of invasion (i.e., in sympatry) and once replacement has occurred (i.e., in allopatry). We compiled data sets (sympatry: n = 169; allopatry: n = 687) from across the historical range of inland cutthroat trout and made comparisons for all sites combined (western scale) and among regions. In sympatry, at the western scale we observed that brook trout exhibited 3.1 times greater density and 2.5 times greater biomass than cutthroat trout, but no difference in production. In allopatry, density and biomass were 1.5 times greater and annual production 1.9 times higher for brook trout than cutthroat trout. Results of comparisons by region were similar, though more variable. In addition, we found that trout density and biomass were correlated with landscape features such as elevation and gradient, but such watershed variables did not account for the differences we observed between the trout species. These results suggest a general pattern of greater density, biomass, and production when brook trout invade and replace cutthroat trout, and highlight the need for more studies that elucidate the mechanisms responsible for this pattern and that investigate the broader consequences of a nonnative species replacing a closely related (and often presumed to be similar) native species.

Advances in variable selection methods II: Effect of variable selection method on classification of hydrologically similar watersheds in three Mid-Atlantic ecoregions

Hydrological flow predictions in ungauged and sparsely gauged watersheds use regionalization or classification of hydrologically similar watersheds to develop empirical relationships between hydrologic, climatic, and watershed variables. The watershed classifications may be based on geographic proximity, regional frameworks such as ecoregions or classification using cluster analysis of watershed descriptors. General approaches used in classifying hydrologically similar watersheds use climatic and watershed variables or statistics of streamflow data. Use of climatic and watershed descriptors requires variable selection to minimize redundancy from a large pool of potential variables. This study compares classification performance of four variable groups to identify homogeneous watersheds in three Mid-Atlantic ecoregions (USA): Appalachian Plateau, Piedmont, and Ridge and Valley. The variable groups included: (1) variables that define watershed geographic proximity; (2) variables that define watershed hypsometry; (3) variables selected using causal selection algorithms; and (4) variables selected using principal component analysis (PCA) and stepwise regression. The classification results were compared to reference watersheds classified as homogeneous using three streamflow indices: Slope of flow duration curve; Baseflow index; and Streamflow elasticity using a similarity index (SI). Classification performance was highest using variables selected by causal algorithms (e.g., HITON-MB method, SI=0.71 for Appalachian Plateau, SI=0.90 for Piedmont, and SI=0.72 for Ridge and Valley) compared to variables selected by stepwise regression (SI=0.72 for Appalachian Plateau, SI=0.87 for Piedmont, and SI=0.64 for Ridge and Valley) and PCA (SI=0.71 for Appalachian Plateau, SI=0.76 for Piedmont, and SI=0.57 for Ridge and Valley).

Logging and turbidity in the coastal watersheds of northern California

Continuous turbidity data for the 2004–2005 winter runoff seasons were used to assess water quality characteristics in 28 coastal watersheds in northern California. Turbidity probes collected data during the winter period, typically spanning the months of October–May. Stream biota, such as salmonids, suffer not only from turbidity extremes but also from chronic turbidity. We used turbidity at the 10% exceedence level to index chronic turbidity in the 28 streams. Watersheds draining to the streams spanned disturbance categories from pristine redwood forest to intensive commercial timber harvest. Grouping the sites by timber harvest history showed that the pristine (unharvested, or ‘background’) group mean was 8 FNU (formazin nephelometric units) at the 10% exceedence level in water year 2005 (WY2005), while the legacy (older) harvest, low, and high harvest rate group means were 16, 32, and 61 FNU, respectively. Regression analyses of turbidity on watershed natural physiographic characteristics and land use histories (logging and roads) showed the rate of recent logging (mean annual percent of watershed area) explained the greatest amount of variability in turbidity at the 10% exceedence level. Drainage area was also significant but was secondary to harvest rate. None of the other watershed variables was found to improve the regression models. Despite much improved best management practices, contemporary timber harvest can trigger serious cumulative watershed effects when too much of a watershed is harvested over too short a time period.

Stream recession curves and storage variability in small watersheds

Abstract. The pattern of streamflow recession after rain events offers clues about the relationship between watershed runoff (observable as river discharge) and water storage (not directly observable) and can help in water resource assessment and prediction. However, there have been few systematic assessments of how streamflow recession varies across flow rates and how it relates to independent assessments of terrestrial water storage. We characterized the streamflow recession pattern in 61 relatively undisturbed small watersheds (1–100 km2) across the coterminous United States with multiyear records of hourly streamflow from automated gauges. We used the North American Regional Reanalysis to help identify periods where precipitation, snowmelt, and evaporation were small compared to streamflow. The order of magnitude of the recession timescale increases from 1 day at high flow rates (~1 mm h−1) to 10 days at low flow rates (~0.01 mm h−1), leveling off at low flow rates. There is significant variability in the recession timescale at a given flow rate between basins, which correlates with climate and geomorphic variables such as the ratio of mean streamflow to precipitation and soil water infiltration capacity. Stepwise multiple regression was used to construct a six-variable predictive model that explained some 80 % of the variance in recession timescale at high flow rates and 30–50 % at low flow rates. Seasonal and interannual variability in inferred storage shows similar time evolution to regional-scale water storage variability estimated from GRACE satellite gravity data and from land surface modeling forced by observed meteorology, but is up to a factor of 10 smaller. Study of this discrepancy in the inferred storage amplitude may provide clues to the range of validity of the recession curve approach to relating runoff and storage.

Hydrological characterization of watersheds in the Blue Nile Basin, Ethiopia

Abstract. Thirty-two watersheds (31–4350 km2), in the Blue Nile Basin, Ethiopia, were hydrologically characterized with data from a study of water and land resources by the US Department of Interior, Bureau of Reclamation (USBR) published in 1964. The USBR document contains data on flow, topography, geology, soil type, and land use for the period 1959 to 1963. The aim of the study was to identify watershed variables best explaining the variation in the hydrological regime, with a special focus on low flows. Moreover, this study aimed to identify variables that may be susceptible to management policies for developing and securing water resources in dry periods. Principal Component Analysis (PCA) and Partial Least Square (PLS) were used to analyze the relationship between five hydrologic response variables (total flow, high flow, low flow, runoff coefficient, low flow index) and 30 potential explanatory watershed variables. The explanatory watershed variables were classified into three groups: land use, climate and topography as well as geology and soil type. Each of the three groups had almost equal influence on the variation in hydrologic variables (R2 values ranging from 0.3 to 0.4). Specific variables from within each of the three groups of explanatory variables were better in explaining the variation. Low flow and low flow index were positively correlated to land use types woodland, dense wet forest and savannah grassland, whereas grazing land and bush land were negatively correlated. We concluded that extra care for preserving low flow should be taken on tuffs/basalts which comprise 52% of the Blue Nile Basin. Land use management plans should recognize that woodland, dense wet forest and savannah grassland can promote higher low flows, while grazing land diminishes low flows.

A multi-scale approach to prioritize wetland restoration for watershed-level water quality improvement

Wetland restoration is commonly presented as an important strategy for maintaining and enhancing the water quality and ecological capital of watershed-scale ecosystems. Prioritizing restoration sites on the landscape is often a haphazard process based on widely held, though often untested, assumptions about relationships between watershed characteristics and water quality. We present a framework to target and prioritize wetland restoration locations using both regional and watershed-level screening models. The regression-tree and random forest models presented in this paper identify watershed variables with the strongest relationships to a given water quality parameter, present a clear hierarchy of variable importance, and present approximate thresholds in watershed area where these variables express the greatest impact on water quality. The proportion of watersheds classified as prior-converted agricultural land was an important predictor of both ortho and total phosphorus. Fortunately because prior-converted agricultural lands were historically wetlands, they are often very suitable for wetland restoration. These sites often have poorly-drained soils requiring artificial drainage to be suitable for agriculture. These drainage systems become conduits for transporting phosphorus from agricultural field and to area streams and rivers. Maintaining natural land-cover within stream buffers is identified as another important predictor of water quality. This seems to be especially true with regard to NO3–NO2 concentrations. Our model results support specific management recommendations including: (a) exclusion of agricultural land-uses from riparian buffers, (b) maintaining or increasing watershed-level wetland-cover and (c) reducing wetland fragmentation.