Understanding Dominant Moisture Transport Tracks of Summer Monsoon 
Rainfall and Space-time Variability of Summer Streamflow over South 
Western U.S

Summer monsoon (Jun-Sep) rainfall contributes substantially to the 
water resources in the semi-arid South Western U.S especially to the 
Lower Colorado River Basin and its tributaries. While winter rain and 
snow contribute over half of the annual flow to these river systems, 
summer precipitation is important for ecosystems. These rains, part of 
the North American Monsoon (NAM), are a result of complex interaction 
between synoptic and mesoscale circulation features coupled with 
varied topography over South Western U.S. The general synoptic 
features during the summer season are well known, but to understand 
the variability better, especially the extreme precipitation, it is 
important to understand the sources and pathways through which 
moisture is delivered to this region during summer. In this study the 
Lagrangian particle trajectory HYSPLIT model is employed at four 
locations in the southwestern U.S. region to identify the dominant 
tracks of moisture. Each location is supplied moisture from Gulf of 
California, Gulf of Mexico, Pacific Ocean or land sources and the 
frequency of trajectories from their respective dominant sources are 
linked to large scale ocean-atmospheric features. During this season, 
more than 85% of the precipitable water lies below 1500m. AGL. For the 
northern half of the monsoon region, a major part of the moisture that 
precipitates out, comes from local evaporation versus horizontal 
transport. Further, more than half of the extreme rainfall events at 
each location are a result of moisture transported from their 
respective dominant sources. 


To complement the above, analysis of summer season streamflow was 
performed. Four distinct spatial patterns emerge – Upper Colorado 
Basin; Rio Grande, Lower Colorado Basin and Upper Gila - and their 
temporal patterns correlate well with their respective regional 
precipitation. The variability in the flow is also well related to the 
moisture tracks and large-scale ocean-atmospheric features, which 
drives the multi-year variability. These physical mechanistic 
connections between large scale climate, moisture transport, 
precipitation and streamflow can be of potential use in enhancing the 
forecast skills.