L Modeling the impact of land cover change and water withdrawals on runoff and N retention in the Ipswich River, MA Hydrological Modeling Nitrogen Loading and Removal DIN inputs to the river network are based on an empirical relationship with land use type and runoff conditions (Wollheim et al. 2008) Removal based on a two-compartment nutrient spiraling model (Mulholland and DeAngelis 2000) and updated via Stewart et al. (In Review) The output flux from a river grid cell becomes the input flux to the cell immediately downstream, and so on downstream for the sequence of grid cells to the river mouth Conclusions Water withdrawals offset the increase in runoff (+28.5%) due to impervious cover, septic inputs, lawns, and lawn watering at annual time scales. Impervious surfaces provide the greatest increase to runoff during annual time periods, and are equally as significant as lawn watering during summer periods. Land cover change generally reduces DIN retention whereas water withdrawals increase apparent DIN retention at basin scales. Network models are necessary to better understand the counter-acting forces of urbanization on water and N fluxes to oceans. University of New Hampshire Water Systems Analysis Group Introduction: Question: What is the impact of urbanization (specifically impervious cover, septic inputs, lawn surfaces, lawn watering, and water withdrawals) on water quantity and DIN retention in the Ipswich River? Rationale: 1)Increases in impervious cover and lawn surfaces have been shown to increase the magnitude of runoff (thereby reducing water residence times) whereas water withdrawals can have a counter-acting effect. 2)These hydrological changes have implications for nitrogen retention because discharge and channel hydraulics influence the duration that stream water is in contact with reactive surfaces. Approach: Use a spatially-distributed, process-based river network DIN removal model that has been populated with established hydrologic, geomorphologic, and biologic parameters to quantify runoff and DIN fluxes under various land cover scenarios. Long Term Ecological Research R.J. Stewart 1 W.M. Wollheim 1, C. Polsky 2, R.G. Pontius 2, C.S. Hopkinson 3 (1) University of New Hampshire, Durham, NH, (2) Clark University, Worcester, MA, (3) University of Georgia, Athens, GA River width and depth are simulated using power law relationships as a function of mean annual Q: Depth MC = 0.45 * Q 0.17 Width MC = 9.56 * Q 0.65 Surface TS Removal STS = 1 – exp (-k t τ STS ) Main Channel Removal MC = 1 – exp (-V f /H L ) STS Transfer = (α STS * A MC * L) / Q HTS Transfer = (α HTS * A MC * L) / Q Downstream DIN Flux Upstream and Local DIN Inputs Hyporheic TS Removal HTS = 1 – exp (-k t τ HTS ) Conceptual Diagram of a Single River Grid Cell Model Scenarios: Ipswich River Network k t = 0.64 d -1 V f = 0.08 m d -1 Precipitation R gw ET R surface Impervious *20% 80% Rooting Zone (AWC) R storm Snowpack Snowmelt Wetland Detention Pool Hamon method 2.γ * surplus 3.(1 – γ) * surplus 4.β * GWpool 5.Φ * Swpool * Pellerin et al Groundwater Detention Pool 4. Lawn R total = R storm + R surface + R gw ET Pervious 6 Parameters γ = infiltration fraction β = groundwater release Φ = surface storage release Ω = soil drying coefficient Snowfall = temp. threshold Snowmelt = temp. threshold A modified version of the Water Balance Model (WBM, Vörösmarty et al. 1998) was applied to the Ipswich River to simulate river discharge on a daily time-step [right panel]. Urban Features Zarriello 2002 (USGS) Archfield et al (USGS) USGS Ipswich USGS Middleton Salem – Beverly Wdl. Peabody Wdl. Lynn Wdl. River Mouth USGS Gauges Water Supply Wdls. Commercial Wdls. Ipswich River Network 024 km 1.) Pristine: no urban features 2.) + Impervious: impervious surfaces only 3.) + Septic: imperious surfaces and septic inputs 4.) + Lawns and Watering: all of the above plus lawns/watering 5.) + Withdrawals: all of the above with water withdrawals k t = time specific uptake rate [T -1 ] A MC = cross sectional area of MC [L 2 ] A HTS = cross sectional area of HTS [L 2 ] Nutrient Spiraling Model Terms A STS = cross sectional area of STS [L 2 ] V f = nutrient uptake velocity [L T- 1 ] L = grid cell length [L] α = exchange rate [T -1 ] H L = hydraulic load [L T -1 ] τ = A TS / (α A MC ) [T] Results Impervious surfaces based on MassGIS data (2007). Septic inputs based on town data and average US domestic water use (waterfootprint.org, 2001). Lawns and watering: Data layers provided by MassGIS and Clark U. Reduced soil rooting depths were applied for lawns (AWC = 25) and lawn watering consists of 1 inch, applied once per week (June, July, August). Water withdrawal schedules and volumes based on USGS data Septic Observed and Predicted Runoff at USGS Ipswich Average Daily Runoff Summary [2000 – 2004] Period Obs. (mm d - 1 ) Pristine (mm d -1 ) +Imp. (mm d -1 ) +Septic (mm d -1 ) +Lawns (mm d -1 ) +Wdls. (mm d -1 ) Annual 1.41 (-) 1.41 (-) 1.58 (12.1%) 1.68 (19.1%) 1.81 (28.4%) 1.36 (-3.5%) Summer 0.84 (-) 0.10 (-) 0.31 (310%) 0.41 (410%) 0.65 (650%) 0.54 (540%) Runoff depth (percent change from pristine scenario) Observation 1.) Pristine (NS = 0.41) 2.) + Impervious (NS = 0.44) 4.) + Lawns/Watering (NS = 0.41) 5.) + Withdrawals (NS = 0.50) Legend 3.) + Septic (NS = 0.43) Scenario 5 + Wdls. Percent of Total DIN Inputs Removed (Summer Average, ) Scenario 1 Pristine Scenario 2 + Impervious Scenario 3 + Septic Percent of Total DIN Inputs Removed Scenario 4 + Lawns/ Watering 93.7% 82.0% 81.0% 71.5% 77.4% Runoff (mm/d) Runoff (mm/d) 0 to 1 m 3 s -1 Modeled (Scenario 5) Observed 1 to 2 m 3 s -1 2 to 3 m 3 s -1 3 to 4 m 3 s -1 Modeled (Scenario 5) ObservedModeled (Scenario 5) ObservedModeled (Scenario 5) Observed Observed and Predicted DIN Concentrations (binned based on discharge at river mouth) DIN (mg L -1 ) Proportion of Local Inputs Leaked from Network (Summer Average, ) Scenario 4 + Lawns/Waterin g Scenario 1 Pristine Scenario 3 + Septic Scenario 5 + Withdrawals Scenario 2 + Impervious Legend National Science Foundation