Responses to climate change

Water budget results

In order to conduct a water balance assessment, the model domain was divided into several water budget zones (Fig. 14), and Zone Budget (ZBUD) was run. ZBUD (McDonald & Harbaugh 1988) calculates sub-regional water budgets using results from MODFLOW simulations. In the top two aquifer layers, ZBUD zones were delineated for all cells. Zone 1 includes the City of Grand Forks (GF) and other areas considered as background to the main irrigation districts. In the five irrigation districts (zones 3 to 7), polygons of irrigated areas (fields) were also used for ZBUD zone delineation, taking into account areas that are irrigated in the large districts. Flow budget zones in these districts only apply to actually irrigated areas, and not whole districts. The river floodplain was given a separate zone (2). It is meant to account for river-aquifer exchanges in the low-lying areas that have head values very similar to river elevations and react very quickly to changes in river water levels. The underlying silt (7) and clay beneath the silt (8, not shown) were similarly assigned as zones.

During spring freshet on the Kettle River, the rise in river stage causes inflow of water to various ZBUD zones (after passing through the floodplain area). This excess water is stored in the aquifer. Mass balance calculations indicate that storage rates are less than 50% of inter-zonal groundwater

Calculated vs. Observed Head Time - ISO days

Observe cl Head :n;

Fig. 13. (a) Wells with static groundwater levels in BC well database, and location of observation well 217 with monthly water records. (b) Residuals at model time 160 (Julian day) from transient model run for 1961-1999 climate.

Observe cl Head :n;

Fig. 13. (a) Wells with static groundwater levels in BC well database, and location of observation well 217 with monthly water records. (b) Residuals at model time 160 (Julian day) from transient model run for 1961-1999 climate.

Fig. 14. Water budget zones (numbers in brackets are the zone numbers referred to in the text) in MODFLOW model of Grand Forks aquifer (top layer shown).

flux, and 15 to 20% of river-aquifer flux. As river stage drops, the hydraulic gradient is reversed and water is released from storage and leaves mostly to the floodplain zone as it returns to the river as base-flow. The rate of inflow of groundwater from the river and into the aquifer along the floodplain zone follows the river hydrograph very closely during the rise in river stage. As the river stage levels off and begins to decrease, the flow direction is reversed within 10 days, and the rate of inflow from the aquifer to the river begins to rise, and then dominates for the rest of the year, as water previously stored in the aquifer drains back to the river as baseflow seepage. As most of the pumping water is lost to evapotranspiration on irrigated fields, there is a small reduction in the baseflow component to the Kettle River during the pumping period.

The river-aquifer interaction has a maximum flow rate of 41 m3/s, which translates to between 11 and 20% of river flow during spring freshet. Thus, the river puts about 15% of its spring freshet flow into storage in Grand Forks valley aquifer, and within 30 to 60 days most of that water is released back to the river as baseflow. During the freshet conditions, 15% of river flow (in m3/s) occurs through the surficial aquifer, mostly in the floodplain area. As the river peak flow shifts to an earlier date in a year for the future climate scenarios, the 'hydrographs' for aquifer water levels also shift by the same interval. Impacts are smallest in zones least connected to the river (away from the river and at higher elevation).

Volumetric recharge accounts for 1 to 7% of other flow components, such as flow between zones and storage. In most zones, recharge increases from winter to summer, then remains high until late autumn and decreases towards the winter months. Irrigation return flow begins after day 150. In zones 4, 5 (shown) and 7, recharge increases by 10 to 20% due to irrigation return flow. Return flow is constant for any climate scenario because it was calculated from present irrigation use of groundwater in each district (return flow component does not increase with climate change even if recharge changes). Figure 15 shows recharge for zone 5, the Big Y Irrigation District, which is the largest irrigation district in the valley. Irrigation return flow can be observed by differences between pumping and non-pumping recharge for any one climate scenario.

The predicted future climate for the Grand Forks area from the downscaled CGCM1 model will result in more recharge to the unconfined aquifer from spring to summer seasons. The largest predicted increase is from day 100 to day 150, when it is predicted to increase by a factor of three or more from present levels, according to ZBUD results from all zones. In the summer months, recharge is also

2010-2039 2040-2069 2070-2099 1961-1999 pumping 2010-2039 pumping 2040-2069 pumping 2070-2099 pumping

0 30 60 90 120 150 180 210 240 270 300 330 360 Model Time (Julian Day)

Fig. 15. Transient model recharge flow volumes for Zone 5 (Big Y Irrigation District as shown in Fig. 14).

2010-2039 2040-2069 2070-2099 1961-1999 pumping 2010-2039 pumping 2040-2069 pumping 2070-2099 pumping

0 30 60 90 120 150 180 210 240 270 300 330 360 Model Time (Julian Day)

Fig. 15. Transient model recharge flow volumes for Zone 5 (Big Y Irrigation District as shown in Fig. 14).

predicted to be approximately 50% greater than at present (in most zones). In the autumn season the recharge is predicted to increase (10 to 25%) or remain the same as present, depending on location in the valley. In the winter the CGCM1 weather predictions suggest less precipitation in winter and, thus, less recharge to the aquifer.

Within an annual cycle and between climate scenarios the results show different spatial and temporal distributions in groundwater conditions. Head difference maps (Fig. 16) were prepared to show differences due to climate change between future climate scenario model outputs and present climate scenario model outputs. At present, the flow patterns are influenced by river channel profile, and generally follow valley floor topography. In this particular aquifer, the effect of changing recharge on ground-water levels is very small compared to changes in timing of basin-scale snowmelt events in the Kettle River and the subsequent shift in hydrograph.

In the 2010-2039 scenario, water levels rise and fall with the river hydrograph at different times because of a shift in river hydrograph peak flow to an earlier date. Elevated water levels up to 30 to 40 cm persist along the channel and drain within a month. From late summer to the end of the year, water levels are similar to present conditions, with small increases observed due to the increase in recharge in areas away from the river channel. Overall, the climate change effects for the 20102039 scenario relative to present are limited to the floodplain, and to the early part of the year when the river hydrograph shifts and is at peak flow levels. A small increase of water levels due to the increase in recharge is forecast for the 2010 climate.

In the 2040-2069 climate scenario, the hydrograph shift is larger than in the 2010-2039 climate scenario, resulting in up to 50 cm change in groundwater levels. Parts of the valley aquifer that are strongly connected to the river have the largest climate-driven changes: the 'hydrographs'

Fig. 16. Water level differences between future (2010-2039) and present climate, and future (2040-2069) and present climate under pumping conditions. Maps by time step in days 131 to 305. Positive contours are shown at 0.1 m interval. The zero contour is a dashed line. Negative contours are not shown. Darkest colours indicate values <—0.5 m (along rivers only). At day 101 (not shown), difference map has values within 0.1 m of zero (Scibek & Allen 2006).

Fig. 16. Water level differences between future (2010-2039) and present climate, and future (2040-2069) and present climate under pumping conditions. Maps by time step in days 131 to 305. Positive contours are shown at 0.1 m interval. The zero contour is a dashed line. Negative contours are not shown. Darkest colours indicate values <—0.5 m (along rivers only). At day 101 (not shown), difference map has values within 0.1 m of zero (Scibek & Allen 2006).

for groundwater levels also shift similarly to the river hydrograph between climate scenarios.

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