Observed 1999 April-July volume flows for rivers in the San Juan Basin were near average compared to below average last year. Based on a sample population of 11 stream gages, spring runoff flows (April -July) ranged from 85%-115% of normal with a median of 100%. No gauged river sites recorded flows above bankfull, subsequently, no flooding occurred in the San Juan Basin during the runoff period.
Precipitation amounts split the water year into extreme halves. The first half being extremely dry and the second half extremely wet. Most of the precipitation fell in April with 240% of average realized, which followed some very dry months. December through March received much below average precipitation (20%, 55%, 45% and 20%, respectively). April through July received much above average precipitation (240%, 140%, 140% and 185%, respectively). The average basinwide snowpack was near average on January 1st and February 1st (100% and 90%), below average on March 1st (85%) and much below average on April 1st (55%). On May 1st , the snowpack had rebound to near average (90%). By June 1st, the snowpack had decreased once again to below average (85%).
The January and February April-July forecasts were below average in anticipation of dry la nina conditions, even though the snowpack was near average. Reports indicated that the snowpack percentages were misleading because of the fact that there was no snow at mid-elevations, which was reflected in the below average forecasts early in the water year. The February forecast increased slightly due to an increase in lower elevation snowpack. However, with continued dry conditions, forecasts dropped to a low of approximately 50% of average in April. Following the onslaught of very wet conditions, forecasts were raised approximately 35% in May and June to 85%. However, much above average precipitation continued into June through July, resulting in near average April-July runoff.