Precipitation and erosion dynamics in the Rio Puerco Basin

Abstract

River basins in arid and semiarid regions, with highly erodible land surface and highly variable precipitation, are susceptible to dramatic watershed and channel erosion. In these environments, even small changes in mean climate can produce a large response in the erosion dynamics. To predict the direction and magnitude of future channel adjustment, it is necessary to understand the processes of erosion and deposition in ephemeral streams and the role of spatial and temporal variability in rainfall and runoff. That is the objective of this study. A modeling analysis of precipitation and erosion dynamics is conducted on the Rio Puerco Basin, New Mexico. Annual, monthly and daily rainfall and runoff data are analyzed for trends in the recent record (1948-1997). Analyses at the annual timescale show a statistically significant increasing trend in Rio Puerco precipitation. This trend is connected with large scale climate patterns and anomalies in the Northern Pacific. Analyses at finer temporal scales show that the increase in precipitation is a result of rising non-summer rainfall, and identify substantial shifts in the daily distributions of both rainfall and runoff. It is postulated that changes in runoff are related to recent alluviation in the Rio Puerco stream system and to vegetation changes in the Basin. The spatial and temporal variability in daily rainfall is investigated by the discrete random cascade theory using radar and climate station data. A rainfall modeling system is developed that disaggregates regional annual rainfall into a sequence of daily stochastic rainfall fields, while preserving important statistical properties at all spatial and temporal scales involved. A conceptual lumped rainfall-runoff model transforms spatially distributed rainfall into daily streamflow on a subbasin scale. A new channel erosion model ERMO is developed and applied to the Rio Puerco main stream. The model is used to analyze channel change in long-term simulations, with special emphasis placed on the role of transmission losses. Modeling with stochastic rainfall shows the importance of external climate factors (average regional precipitation, spatial distribution and magnitude of rainfall, etc.) and internal channel characteristics (tributary confluences, distribution of channel width, infiltration into the streambed, etc.) on the erosion dynamics. It is determined that although aggradation is the dominant process in the Rio Puerco main stream in the long term, short term variability is caused mostly by rapid degradation during wet years. Particular attention is given to general trends in channel change under a changing climate. It is found that moving towards a wetter and more extreme climate generally brings more stream-wide degradation and an intensification of both maximum erosion and deposition rates.