Swamp Timothy production response to a modified hydrology in wetlands of the grassland ecological area

Habitat restoration efforts involve the risk of imposing negative effects on adjacent ecosystems. Investigations in adaptive management strategies can assist in determining the magnitude of risk associated with restoration strategies. The Lower San Joaquin River (LSJR) can experience very low flow conditions, offering little to no flows to dilute LSJR watershed drainage in critically dry years. The Central Valley Regional Water Quality Control Board has adopted a conditional waiver of Waste Discharge Requirement for discharges from irrigated lands, which requires the characterization of wetland water quality, and a salt and boron Total Maximum Daily Load, limiting the amount of salt and boron that can be discharged into the San Joaquin River at times of limited assimilative capacity. Wetland managers are being asked to modify their normal water management by draining wetlands to match sufficient assimilative capacity in the LSJR to dilute saline drainage discharges to meet load allocations. Wetland managers are concerned that altering the normal hydrology in managed wetlands will adversely impact the productivity of these wetlands over time. A drainage strategy that has been proposed is a 4-6 week delay in drainage of wetlands managed for swamp timothy. The schedule of the delay drawdown was chosen to coincide with capacity in the LSJR associated with the State Water Resource Control Board's Vernalis Adaptive Management Program, VAMP. Twelve impoundments from six properties were selected to represent the spectrum of swamp timothy managed wetlands in the GEA. Treatment fields from each property were randomly selected to undergo a 4-6 week delay in spring drawdown and a subsequent lack of irrigation. The remaining six control fields were managed normally with a mid March drawdown and normal 4 week post drawdown irrigation. Despite the large variation found in annual swamp timothy seed and biomass production a significant treatment by time interaction was observed using a repeated measures analysis. Although a reduction in seed and biomass yield was observed in many fields over the study duration due to natural year to year production variation, delayed drawdown treatment fields declined to a greater magnitude than normally managed control fields at all properties.