San Juan Basin
Observed April through July volume flows for rivers in the San Juan
Basin were much below average. Based on a sample population of eight stream
gages, spring runoff flows ranged from 30 to 60% of normal with a median of
45%. On January 1st water year precipitation and snow pack across the San Juan
were much below average at 30 and 35%, respectively. During the first two
months of 1996, monthly precipitation across the San Juan was much above
average (130%). This helped to raise March 1st water year precipitation and
snowpack from much below average to below average values. Monthly
precipitation dropped to below normal during March (70%) and April (80%) and
was much below average for May (30%). Water year precipitation remained steady
at approximately 70% of normal from April 1st to June 1st. Snowpack declined
from its high on March 1st (80%) to 60% by May 1st. Monthly flows for the
runoff period were highest during May and lowest in July. In summary, below to
much below normal precipitation across the San Juan produced much below average
flows occurring earlier in the season than normal.
Based on early season snow accumulation, water year precipitation, and
streamflow observations, January and February volume forecasts for the April
through July period were much below average. These were raised slightly in
March to account for increased precipitation, but later dropped in April as the
snowpack was reduced during March. May forecasts were further reduced as it
became apparent that snowpack had peaked for the season and was rapidly melting
due to increasingly warmer daytime temperatures. May forecasts slightly
overpredicted observed runoff volumes (5% overprediction, on average, based on
eight forecast points). Observed flows were within the reasonable maximum and
minimum confidence bounds for all forecasts.
