Water pollution in response to accelerated land-use/land cover changes has drawn concerns because of public health and environmental impacts. The study was conducted to evaluate the impact of land use/land cover changes, seasonal, and location on water quality of streams within the Wheeler Lake Basin in northern Alabama. Temporal water samples from 18 streams were randomly collected in 2000 and 2001, processed and analyzed for pH, and total nitrogen (TN), dissolved (Dp), particulate (Pp) and total phosphorus (Tp), dissolved oxygen (DO) and soluble lead (Pb) concentration, employing standard methods of analysis. The data were normalized and integrated into a simple index (WQC Index ) to evaluate stream water quality. Results showed that the urban proportion of the total watershed basin had increased from 2.9 to 14.7% with an associated loss of agricultural (8.9%) and wetland (4.8%) covers from 1992 to 2000. A change in land-use/land covers in association with seasonal and location variation significantly affected stream water quality. Total nitrogen concentration in stream water had a peak during the summer at 34% above the annual mean. While both Pp and Tp concentrations peaking during the summer at 24% above the annual mean and about 25% below the annual mean during spring, the DO concentrations were 46% above the annual mean during the fall and 18 to 26% below annual mean during summer. The WQC Index had responded very seasonal and showed significant identical trends, with 21% degradation in water quality during the summer above the annual mean and improvement during the spring at 20% above the annual mean. Upstream water had a significantly greater Pp and Tp (21 to 28%) concentration than at down-and middle streams water. Location and seasonal variations had significant interactive effects on Pp, Tp and DO concentration of stream water. Total amount of seasonal rainfall significantly accounted 99.6% of the variations in WQC Index . Increasing seasonal mean relative humidity, air and soil temperature, evaporation and solar radiation had positive relationship with the variations in WQC Index . Among the water quality parameters, both Pp and Tp were correlated (r 2 = 0.998 * * * ) to each other, and accounted for more than 80% variability of the WQC Index . Highly significant positive linear relationship between Pp and Tp concentration suggested that 99.8% of the P in stream water is in Pp form which probably transported with sediments in surface runoff. In other words, Pp is the main pollutant responsible for degradation of stream water quality in the Wheeler Lake Basin. Routine measurement of either Pp or Tp concentration could be used as sensitive and early indicator of temporal changes in stream water quality even when the other parameters changed negligibly or remain unchanged.
The rational method for peak discharge (Q p) estimation was introduced in the 1880s. Although the rational method is considered simplistic, it remains an effective method for estimating peak discharge for small watersheds. The runoff coefficient (C) is a key parameter for the rational method and can be estimated in various ways. Literature-based C values (C lit) are listed for different land-use/land cover (two words, no hyphen) (LULC) conditions in various design manuals and textbooks; however, these C lit values were developed with little basis on observed rainfall and runoff data. In this paper, C lit values were derived for 90 watersheds in Texas by using LULC data for 1992 and 2001; the C lit values derived from the two data sets were essentially the same. Also for this study, volumetric runoff coefficients (C v) were estimated by using observed rainfall and runoff depths from more than 1,600 events observed in the watersheds. Watershed-median and watershedaverage C v values were computed, and both are consistent with data from the National Urban Runoff Program. In addition, C v values were estimated by using rank-ordered pairs of rainfall and runoff depths (i.e., frequency matching). As anticipated, C values derived by all three methods (literature based, event totals, and frequency matching) consistently had larger values for developed watersheds than for undeveloped watersheds. Two regression equations of C v versus percent impervious area were developed and combined into a single equation that can be used to rapidly estimate C v values for similar Texas watersheds.
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