Shawn Kuzara - Montana AWRA

GROUNDWATER SURFACE WATER INTERACTION
IN THE ALLUVIAL AQUIFER:
MODELING THE MIDDLE STILLWATER RIVER VALLEY
By
Shawn Kuzara
10‐7‐2011
Funding
Renewable Resources Grant (DNRC)
Sponsored by the Stillwater Conservation District
Study area location
USGS station
Stillwater River
Study Area
Semi‐arid climate
Average annual precipitation ~ 18 inches
~3,200 acres of irrigated land
Tributary streams (dry to 4 cfs)
8 ditches that serve the area
Measured ditch flow range of 4 to 33 cfs Average alluvial thickness of ~30 ft
Average saturated thickness between 21 and 26 ft
Geology
Seasonal changes in groundwater levels
1500 ft
Steady State Groundwater Model
Finite difference method using MODFLOW
‐ Layer‐Property Flow (LPF) package
‐ 3 layer model
‐ Head‐dependent flow boundaries general head, river
N
1 Mile
Calibration
Three targets accomplished
Water table map
Flow Budget
IN:
Flow
(ft3/day)
Flow (cfs)
River Leakage
General Head Boundary
Recharge
Total In
OUT:
17,909
873,509
128,250
1,019,668
0.21
10.11
1.48
11.80
River Leakage
General Head Boundary
Recharge
Total Out
973,853
45,809
11.27
0.53
1,019,663
11.80
Percent Discrepancy
0.0
Observation heads 95% within 10 feet of observed water level
Sensitivity
Heads were sensitive
to (GHB, River, and RCH)
Degree depended upon location
Transient Groundwater Model
24 month simulation
Calibration
Observation heads 75% within 10 feet of observed water level
Calibration
Head
Predictive Simulations
Pre‐irrigation
Simulating a time before irrigation existed and precipitation was only from of recharge
Jan
July
July
Jan
Full Seasonal Irrigation
Jan
July
July
Jan
Irrigation vs. non-irrigation
(only precipitation)
‐ Head decrease of up to 18 ft without irrigation ‐ Average of 7cfs less groundwater enters river in dry months (July‐Sept) Jan
July
July
Jan
Head response to ditch leakage
(no flood irrigation)
Jan
July
July
Jan
Questions?
Recharge polygons
Precipitation = 5% of monthly value Center pivot = 0.25 inches/month (June‐Sept) Ditch leakage = 5 inches/month (June‐Sept) Flood irrigation = 9 inches/month (June‐Sept)