Model runs show larger downstream flows when anthropogenic impacts are removed; team explains baseline vs ‘unimpaired’ scenarios

At a technical workshop hosted by the State Water Resources Control Board, consultants showed side‑by‑side results from two model runs: a baseline (historical climate, diversions, irrigation, reservoir operations and pumping) and an “unimpaired” scenario that removes anthropogenic operations but keeps current land use. John Riverson, a consultant with Paradigm Ult, summarized the approach and said the unimpaired run is intended to isolate regulatory/operational impacts rather than represent a fully naturalized landscape.

Riverson said the unimpaired scenario ‘‘turns off the diversions, pumps and major infrastructures like reservoirs and ditches’’ while retaining the most recent land use, and then the team compares time‑series at six locations along the routing network. The graphs shown at the workshop indicate clearly larger flows immediately downstream of the removed reservoir and generally higher flows in unimpaired runs; however, by the time water reaches the watershed mouth much of the water captured for irrigation in mid‑basin is rerouted back as return flows and the percentage change versus baseline is smaller.

The presenters emphasized how the coupled model computes differences: LSPC runs at hourly time steps and its aggregated monthly GUI boundary conditions are passed to Modflow; Modflow returns monthly groundwater–stream exchange terms and LSPC applies a monthly delta distributed across the hourly signal so that the routed streamflow (RO) time series reflects the coupling. Riverson noted that the unimpaired versus baseline comparison helps assess the role of anthropogenic operations on long‑term flow patterns, and that the team ran checks to ensure the simulated water budgets are consistent.

The team reminded stakeholders that scenario assumptions matter: ‘‘unimpaired’’ in this package does not equal ‘‘natural’’ (which would require land‑use change). They invited reviewers to examine the baseline inputs (for example, a reported average 164,000 acre‑feet per year of surface irrigation withdrawals that the team converted to about 227 cfs for reference) and to supply data or corrections during the public comment period.