SIMULATION OF THE SEDIMENT LOAD IN RIO PUERCO, NEW MEXICO BASED ON LINEAR REGRESSION OF 50-YEAR DATA

The Rio Puerco, in central New Mexico, is an ephemeral stream with a drainage area of 7,340 square miles. Most of its flow is runoff from rainfall induced by thunderstorms. The Rio Puerco basin has one of the highest sediment loads in the United States. Furthermore, the sediments contain toxic heavy metals such as arsenic, mercury, and uranium. The high and toxic sediment load damages the stream ecosystem.

The objective of this study is to estimate the sediment load in Rio Puerco using linear regression. Specifically, sediment load, a dependent variable, is estimated as a function of water discharge, an independent variable, using logarithmic regression equations. Sediment load is estimated on a daily, monthly, and annual basis. Data used in this study are published by the U.S.G.S. The data, which were recorded at the gaging station near Bernardo, covered two periods. The first extended for 32 years, between 1947 and 1979, and the second for 15 years, between 1980 and 1994.

Results of the regression analysis are obtained using the SPSS program. The correlation coefficients of the daily, monthly, and annual relationships of the first data set are 0.95, 0.97, and 0.76, respectively. The equivalent values for the second data set are 0.87 (daily), 0.93 (monthly), and 0.90 (annual). Differences in the correlation coefficients between the two data sets are due to the duration of each set, the fact that sediment load in Rio Puerco has decreased over time, and the completeness of each data set. The latter is explained by the U.S.G.S. as follows. Runoff events were adequately sampled only in the first data set. In both data sets, monthly correlation coefficients are higher than daily coefficients since variations in the daily load smooth out when daily loads are added to give monthly values.

The monthly correlation coefficients are the highest in each data set. For this reason, monthly sediment load in Rio Puerco can be predicted as a function of water discharge with a high degree of accuracy. Prediction on an annual basis is not as accurate simply because the Rio Puerco is an ephemeral stream with many dry months in the year. The number of dry months varies considerably from year to year resulting in significant variation in the annual sediment load. The predicted sediment loads compare very well with the observed values.