Study area & diversion structures
SNOTEL (Park Reservoir) — inches
Peak SWE and total water-year precipitation.
Streamflow & diverted volume — acre-feet
Gaged annual volume at both USGS gages, plus diverted volume from the structures upstream of each gage. Diverted volume exceeds gaged flow because a reservoir's fill and a downstream structure's later re-diversion of the same released water are each recorded as separate diversions.
Runoff ratio — dimensionless
Gaged and diverted volume, converted to an equivalent depth over each gage's drainage area, divided by total precipitation. Gaged-only ratios (0.1–0.8) are consistent with a semi-arid mountain watershed. SURCEDCO's own drainage shows greater runoff efficiency than the area added between the two gages — see Methodology for the terrain and soil comparison. The gaged-plus-diverted ratio exceeds 1 in wetter years from the re-diversion effect noted above.
Water-rights priority & size effects (Spearman ρ, 1979–2025)
Colorado water rights are prior-appropriation: in a dry year, senior rights get their full decreed amount before junior rights see any water, and a large decreed amount doesn't automatically insulate a structure from a small snowpack. This asks whether that legal protection actually shows up in the data — do senior and large-decree structures' diversions stay steady regardless of snowpack while junior and small ones swing up and down with it? For this 45-structure subset with a complete record across all 47 years, large/senior structures do correlate with SWE more weakly than small/junior ones in two of the three rankings.
Monthly SWE & precipitation (Park Reservoir) — inches
End-of-month snow water equivalent and monthly precipitation, 2016–2025. Scroll or pinch to zoom into a shorter window, drag to pan.
Monthly soil moisture (Park Reservoir) — % VWC
Monthly mean volumetric water content at 2, 8, and 20 inch depths, 2016–2025. The 20-inch sensor stopped reporting after October 2024. Scroll or pinch to zoom into a shorter window, drag to pan.
Monthly streamflow — cfs
Monthly mean discharge at both gages, 2016–2025. SURCEDCO reports seasonally rather than year-round, so its line has winter gaps. Scroll or pinch to zoom into a shorter window, drag to pan.
Snowmelt-to-streamflow lag (1979–2025)
Monthly snowmelt (the month-over-month decline in SWE) cross-correlated against streamflow at 0–3 month lags. The relationship is strongest in the same month and collapses within a month or two — consistent with this being a small, steep basin with a short time of concentration.
Methodology
Each diversion structure was assigned a position relative to the two gages (upstream of both, between them, or downstream of both) using flowline routing through the USGS 3D Hydrography Program network. Recorded water source reflects legal water-right designation, not necessarily physical location, and was not used for this routing.
Annual values are aggregated by hydrologic year (November 1–October 31), aligning each winter's snow accumulation with the runoff season it produces. Diverted volume reflects DWR's recorded annual figure per structure; a reservoir's fill and a downstream structure's later re-diversion of the same released water are each recorded independently, so total diverted volume exceeds gaged streamflow. Runoff ratios convert gaged and diverted volume to an equivalent depth using each gage's USGS drainage area for direct comparison against SNOTEL precipitation.
Water-rights priority and size effects were evaluated over 1979–2025, the period covered by the SNOTEL station's own record. Not every structure has a diversion record across that full period, so this analysis uses the 45 (of 88) structures with a complete record in every year, comparing the same set of structures from year to year rather than a count that shifts with reporting gaps. Correlations are reported using Pearson, log-transformed Pearson, and Spearman methods.
SURCEDCO's and SURACECO's contributing drainage areas were delineated from a 30-meter DEM (USGS 3DEP) using D8 flow routing, validated against USGS's published drainage areas for these gages (09143000 and 09143500: 27.4 and 38.5 square miles) to within 2 percent — 27.6 and 39.2 square miles as delineated here. SURCEDCO's own drainage is steeper than the area added between the two gages (median slope 8.1° vs. 6.9°) and includes more shallow-to-bedrock terrain (gSSURGO puts roughly a third of it within 25–127 inches of bedrock, versus no bedrock within the surveyed 152-inch profile for the dominant soils downstream) — both favor faster runoff over infiltration. Soil hydrologic group runs the other way: Group C soils, generally associated with higher runoff potential, actually cover a larger share of the added area (86 percent) than SURCEDCO's own drainage (48 percent), so soil type alone doesn't explain the pattern in the runoff ratio chart.