The assessment of periodic changes in water quality and uniform control standards application has been difficult to attain. A potential solution to this problem is the utilization of water quality indices for preliminary screening of data and identification of geographic areas, chronological periods, and possible source and cause of water quality changes. The purpose of this investigation was to demonstrate a practical use and application of a water quality index in Nevada as an existing option for those who must assess water quality changes on a continual basis. The study examined water quality trends in portions of the Colorado, Humboldt, Snake, Walker, and Carson River Basins, Nevada, through application of an existing water quality index to historical data from control point and adjacent monitoring stations. Index ratings were responsive in characterizing changes in water quality status relative to seasonal trends and geographic location. Identification of average annual and single value control criteria violation in terms of which parameter and degree of violation was facilitated through index application. Frequency and magnitude of impact on overall water quality status was also identified. Future use of indices in this manner should prove a valuable tool in meeting legislative water quality management strategies.
Little quantitative site‐specific infiltration, runoff and sediment transport data for Tahoe Basin soils under varying storm events or stage of development are available. Modular (Ml), F‐type (M2), Impact nozzle (M3), and Impact‐Fan nozzle (M4) rainfall simulators were evaluated as to their practicality and ability to characterize infiltration for the Cagwin Soil Series within the Tahoe Basin. Three slope (0–15,15–30, >30%) and four plot conditions (natural with duff [P1], natural without duff [P2], disturbed without duff [P3], and disturbed with duff [P4]) were studied. The measured data were incorporated into a modified Philip's infiltration model and multiple non‐linear regression analyses were used to examine relationships between method, slope, plot condition, and infiltration characteristics.t Simulation methods Ml and M4 produced statistically similar (P=0.01) infiltration data, as did M2 and M3 which produced lower infiltration rates. All were found suitable for use in Sierra Nevada watersheds. Ml was considered most practical. Slope had negligible effect on infiltration. The plot condition was found to significantly influence infiltration, and the effect of each plot condition was significantly different. Final infiltration rates ranged from 4.7 to 6.2 cm/hr. Thus, the Cagwin soil demonstrated moderate to high infiltration rates even when exposed to extreme storm conditions (8–10 cm/hr).
Little quantitative site‐specific infiltration, runoff, and erosion data for forested watershed soils of the Tahoe Basin are available. A Modular‐type rainfall simulator was used to examine these variables for the Cagwin (mixed Typic Cryopsamment) soil series. Three slope (0–15, 15–30, >30%) and four plot conditions (undisturbed with natural duff, undisturbed without natural duff, disturbed without natural duff, and disturbed with natural duff removed) were studied. Infiltration and runoff data were incorporated into a modified Philip's model whereas erosion data were incorporated into a general nonlinear model. Data sets were analyzed via nonlinear regression for slope and plot interaction. Slope had negligible effect on infiltration and runoff but had a significant effect on erosion. Plot condition had significant effects on infiltration, runoff, and erosion. Final infiltration rates ranged from 4.7 to 6.1 cm h−1, runoff ranged from 36 to 59% of the application rates, and cumulative interrill erosion ranged from 37.5 to 108.4 g m−2 for a simulated design storm of 8 to 10 cm h−1. The findings of this investigation were consistent with those of related quantitative investigations and indicate that the Cagwin soil has a low relative erosivity. Data from this study further suggest that previously applied models used to estimate potential erosion hazards of forested watershed soils may well result in over‐estimation of erosion potential.
Water quality indexes permit an evaluation of water quality conditions in comparative terms. This paper presents a methodology for derivation of a generalized water quality index based on factor analysis (a multivariate statistical technique), and examines the geographical applicability of such an index. Ten pollutant parameters from water quality data accumulated in the Carson Valley of Nevada during the 1974, 1975, and 1976 irrigation seasons provided the basis for two indexes: i) WQITN = f(temp., BOD, TP, EC, DODP); and ii) WQIPN = f(temp., BOD, PO4‐P, EC, DODP). Regression analysis of the indexes as functions of water quality variables from two river basins in Nevada indicated WQITN to be the better index (F‐test 99%, R2 = 0.9098). The WQITN formulation was then compared with index values suggested by water quality experts (WQIE). Regression coefficients were determined for WQIE and WQITN index regression equations as functions of water quality data from selected locations throughout the United States. The proposed WQITN appeared the most geographically acceptable index (F‐test 99%, R2 = 0.9754). The model estimated by factor analysis was shown to have good potential as a generalized water quality index. Use of the suggested methodology for the development of generalized, as well as specific‐use water quality indexes, may enable better geographic identification of problem areas contributing to water pollution and the development of appropriate water quality standards. Standardization of the indexes in terms of a range of acceptability, e.g., light, moderate, or severe pollution, is suggested for further investigation.
Pollutants from agricultural irrigation practices continue to require empirical definition. This paper reports results from a 3‐yr investigation of pollutants and pollutant loads carried in irrigation and surface return flows from grass pasture (Bromusinermis Leyss, Dactylis glomerata L., Poa pratensis L., Festuca arundinacea Schrib.) and alfalfa hay lands (Medicago sativa L.) to adjacent receiving waters. Irrigation and surface return flows were metered and the volume of infiltrated water determined at four sites during three irrigation seasons. Concentrations of total dissolved solids (TDS), 5‐day biochemical oxygen demand (BOD5), nitrate‐N (NO3−‐N), total N (TN), phosphate‐P (ortho PO43−‐P), and total P (TP) were combined with flow volumes to compute changes in surface loading per individual irrigation and cumulative throughout the irrigation season. Of those parameters studied, BOD5 and P (PO43−‐P, TP) were found to have a net loss from the fields via surface return flow. Statistical relationships among the water quality parameters varied seasonally. Differences were attributed to seasonal changes in irrigation water supply and availability. Levels of constituent contributions to surface runoff appeared to be more related to site characteristics, water management practice, and hydrologic variation than to the presence or absence of grazing animals. Future efforts in water quality management and control should be directed towards quantifying the relationship between water quality as affected by specific management activities, followed by appropriate economic impact assessment of the control measure effects on agricultural productivity.
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