This study was done to improve the understanding of trends in streamflow of the San  Pedro River in southeastern Arizona. Annual streamflow of the river at Charleston, Arizona, has decreased by more than 50 percent during the 20th century. The San  Pedro River is one of the few remaining free-flowing perennial streams in the arid Southwestern United States, and the riparian forest along the river supports several endangered species and is an important habitat for migratory birds.

Trends in seasonal and annual precipitation and streamflow were evaluated for surrounding areas in southeastern Arizona and southwestern New  Mexico to provide a regional perspective for the trends of the San  Pedro River. Seasonal and annual streamflow trends and the relation between precipitation and streamflow in the San  Pedro River Basin were evaluated to improve the understanding of the causes of trends.

There were few significant trends in seasonal and annual precipitation or streamflow for the regional study area. Precipitation and streamflow records were analyzed for 11  time periods ranging from 1930 to 2002; no significant trends were found in 92 percent of the trend tests for precipitation, and no significant trends were found in 79  percent of the trend tests for streamflow. For the trends in precipitation that were significant, 90 percent were positive and most of those positive trends were in records of winter, spring, or annual precipitation that started during the mid-century drought in 1945-60. For the trends in streamflow that were significant, about half were positive and half were negative.

Trends in precipitation in the San Pedro River Basin were similar to regional precipitation trends for spring and fall values and were different for summer and annual values. The largest difference was in annual precipitation, for which no trend tests were significant in the San Pedro River Basin, and 23 percent of the trend tests were significantly positive in the rest of the study area. Streamflow trends for the San Pedro River were different from regional streamflow trends. All seasonal flows for the San Pedro River, except winter flows, had significant decreasing trends, and seasonal flows for most streams in the rest of the study area had either no trend or a significant increasing trend. Two streams adjacent to the San Pedro River Basin (Whitewater Draw and Santa  Cruz River), however, had significant decreasing trends in summer streamflow.

Factors that caused the decreasing trends in streamflow of the San  Pedro River at Charleston were investigated. Possible factors were fluctuations in precipitation and air temperature, changes in watershed characteristics, human activities, or changes in seasonal distribution of bank storage. This study statistically removed or accounted for the variation in streamflow caused by fluctuations in precipitation. Thus, the remaining variation or trend in streamflow was caused by factors other than precipitation.

Two methods were used to partition the variation in streamflow and to determine trends in the partitioned variation: (1) regression analysis between precipitation and streamflow using all years in the record and evaluation of time trends in regression residuals, and (2) development of regression equations between precipitation and streamflow for three time periods (early, middle, and late parts of the record) and testing to determine if the three regression equations were significantly different. The methods were applied to monthly values of total flow (average flow) and storm runoff (maximum daily mean flow) for 1913-2002, and to monthly values of low flow (3-day low flow) for 1931-2002.

Statistical tests provide strong evidence that factors other than precipitation caused a decrease in streamflow of the San Pedro River. Factors other than precipitation caused significant decreasing trends in streamflows for late spring through early winter and did not cause significant trends for late winter through early spring. Total flows had significant decreasing trends in June through December, low flows had significant decreasing trends in May through December, and storm runoff had significant decreasing trends in July through September. The effects of factors other than precipitation were tested only for July through October for storm runoff.

Besides fluctuations in precipitation, the principal factors that could have caused decreasing streamflow trends are (1) changes in watershed characteristics such as changes in riparian vegetation, changes in upland vegetation, and changes in stream-channel morphology, and (2) human activities such as ground-water pumping, construction of runoff-detention structures, urbanization, and cattle ranching (grazing).

Changes in upland and riparian vegetation likely were major factors in the decreasing trends in total streamflows and low flows. Total flows and low flows in summer and fall were significantly affected by factors other than precipitation, but late winter flows were not significantly affected. The significant effects coincide with high rates of transpiration from vegetation in the summer and the nonsignificant effects coincide with low rates of transpiration in the late winter. Another piece of evidence that implicates vegetation as a cause is that the upland and riparian vegetation of the San  Pedro River Basin changed during the 20th century. The  relative proportions of different species changed in upland vegetation (woody plants increased and grasses decreased), and the areal extent and density of riparian vegetation increased substantially.

Ground-water pumping in the United States and Mexico had a mixed influence on streamflow trends at Charleston, Arizona; statistical analyses indicate that seasonal pumping from wells near the river for irrigation in the spring and summer was a major factor in the decrease in low flows and that year-round pumping from wells in the regional aquifer away from the river was not a major factor in the decrease in low flows. If regional pumping had caused a trend, the pumping should have affected low flows for all months of the year, but factors other than precipitation did not cause significant trends in low flows for January, February, March, and May. Most of the local pumping near the river was during the spring and summer, and this seasonal pumping probably caused some decreases in summer low flows. These conclusions are for trends from 1913 to 2002, and regional pumping in the United States and Mexico could affect streamflow at Charleston in the future, because regional ground-water pumping often has a delayed effect on streamflows.