A SIMPLIFIED WATER QUALITY MODEL FOR WETLANDS

The purpose of this study is to develop a simplified mathematical model to simulate suspended solids and total phosphorus concentrations in a wetland or detention pond. Field data collected from a wet detention pond during storms were used to demonstrate the application of this model. Favorable agreements between the model results and data were achieved. The ratio of average outlet method and summary of loads method were used to quantify the removal efficiency of pollutants, reflecting the efficiencies are very close. The results of this study can be used for nonpoint source pollution control, wastewater treatment or best management practices (BMPs) through the wetland.     

基于连续小波变换的大地电磁信号谱估计方法

在基于连续小波变换的大地电磁信号谱估计方法中 ,通过引入整体平均、小波系数收缩和显著性检验等统计技术 ,以提高谱估计的精度 .文中同时讨论了连续小波变换中各种参数的选取问题 ,给出了Morlet小波函数中尺度与傅里叶频率之间转换的经验公式 ,并给出了谱估计的具体算法 .结果表明 ,本文方法可有效压制较强的白噪声和局部相关噪声 .与FFT谱估计方法相比 ,该方法大大降低了对信号记录长度的要求 ,因而对大地电磁信号的处理有实际意义 .     

CHOICE OF SAMPLING STRATEGY AND ESTIMATION METHOD FOR CALCULATING NITROGEN AND PHOSPHORUS TRANSPORT IN SMALL LOWLAND STREAMS

As reliable estimates of stream nutrient transport are required for many purposes including trend analysis, mass balances and model development, the impact of sampling strategy and estimation method on the bias and precision of stream nitrogen (N) and phosphorus (P) transport calculations was evaluated. The study was undertaken in two catchments in eastern Denmark. Selection of the most accurate sampling strategy and estimation method, i.e. with the lowest root mean square error (RMSE) was based on random (Monte Carlo) runs for generating replicate data sets from an essentially complete record of the concentration of total N (TN), total P (TP), particulate P (PP) and dissolved P (DP) during a two-year period (June 1987 to June 1989). The evaluation comprised 13 different estimation methods and seven discrete sampling strategies involving three categories (regular, stratified and strata sampling). The regular sampling strategies were more accurate (lower RMSE) during high-flow periods than stratified sampling. The greatest improvement in RMSE for TN, TP, PP and DP transport was obtained when increasing the sampling frequency from 12 each year (monthly) to 18 (monthly in summer and fortnightly in winter) and 26 each year (fortnightly). The increase in accuracy (RMSE) was less when increasing the sampling frequency to 52 (weekly) or 104 (biweekly). Nearly all the methods evaluated underestimated the annual transport of TP and PP, whereas TN and DP were both under- and overestimated. The best method of estimating N and P transport when utilizing discrete sampling was both site- and time-dependent. The overall best and most reproducible (stream to stream, year to year) method for estimating annual transport of TN, TP, PP and DP was a linear interpolation method. When this method was used to derive estimates of annual TN and TP transport based on fortnightly sampling, the RMSE was 1.4–5.4 and 20.2–38.5%, respectively, in the Gelbæk stream and 1.1–4.9 and 10.5–15.0%, respectively, in the Gjern Å stream. Subdividing the hydrograph into two strata (low-flow and high-flow periods) and sampling these strata separately for calculating TP transport was superior to discrete sampling for the smaller of the two catchments. A combination of regular sampling (monthly) and pooled high-flow sampling (eight events out of a total of 43) reduced the RMSE of the annual TP load to 10.4%.