Attachment 2
Use of a 7-day Moving Mean to lift Rationing on
Waimea Plains
It is proposed to use a 7-day moving mean flow trigger to recommence pumping at
the end of a period of rationing on the Waimea Plains. This would apply to those
who have not subscribed to the Waimea Community Dam. Once the 7-day moving
mean at the Irvines recorder reaches 6 cumecs (6,000 l/s), pumping from aquifers
could recommence.
Rationale
The principal problem of water supply on the Waimea plains is that of storage, with
aquifers providing most of the water for the irrigators and domestic water users of the
Waimea Basin. Over the length of the river system from the Wairoa Gorge through
to Appleby Bridge, the Waimea River loses up to 1,100 l/s to aquifers. The lower the
aquifers are, the greater the loss of river water to the aquifers, and in 2001 these
losses were enough to cause the Waimea River at Appleby Bridge to dry completely.
After a period of rain during a drought, the resultant rise in river flow provides
immediate relief to the river itself, but the aquifers are slow to respond. They require
either very high levels in the river for a shorter period to force water into the
groundwater system, or an extended period of moderate river flows to allow time for
water to transfer to the aquifers, and storage to build.
If the fresh or flood in the Waimea River is not substantial enough to provide some
recharge to the aquifers, the problem of reduced storage remains. Short term
pumping from aquifers during a flood only makes the storage problem worse. The
low aquifer levels will continue to cause a substantial loss from the river, and in
addition, the danger of saltwater intrusion continues.
The practical effect of the proposed trigger is to allow recommencing of pumping
almost immediately if a moderate or large flood occurs. It will also allow
recommencement of pumping if a series of small peaks occurs over several days. It
will not allow pumping if the flood is either too small, or is short in duration.
It should be noted that the trigger doesn’t take seven days to release irrigators from
rationing. By way of example, an average flow of 14 cumecs over three days will
meet the threshold, as would an average flow of 42 cumecs for one day, or an
average flow of 168 cumecs for six hours.
Examples of river flow plots are shown below. These show the instantaneous river
flow in blue, the 7-day moving mean flow in red, along with a threshold line in black at
a flow of 6 cumecs. Resumption of take would occur when the red line intersects the
black line.
Example 1 – Single peak flood threshold just reached 3 days later
50.0
45.0
40.0
35.0
30.0
25.0
20.0
15.0
10.0
6 cumecs
5.0
0.0
27-Jan-1998
29-Jan-1998
31-Jan-1998
27-Jan-1998 00:00:00 to 6-Feb-1998 00:00:00
2-Feb-1998
4-Feb-1998
Flow (m3/sec) at HY Wairoa at Irvines
MovingMean7Day at HY Wairoa at Irvines
Example 2 – Single peak flood, threshold not reached
50.0
45.0
40.0
35.0
30.0
25.0
20.0
15.0
10.0
6 cumecs
5.0
0.0
12-Apr-2010
14-Apr-2010
16-Apr-2010
12-Apr-2010 00:00:00 to 22-Apr-2010 00:00:00
Flow (m3/sec) at HY Wairoa at Irvines
MovingMean7Day at HY Wairoa at Irvines
18-Apr-2010
20-Apr-2010
Example 3 – Double peak flood, threshold reached 2 days later
50.0
45.0
40.0
35.0
30.0
25.0
20.0
15.0
10.0
6 cumecs
5.0
0.0
18-Mar-2002
20-Mar-2002
22-Mar-2002
18-Mar-2002 00:00:00 to 28-Mar-2002 00:00:00
24-Mar-2002
26-Mar-2002
Flow (m3/sec) at HY Wairoa at Irvines
MovingMean7Day at HY Wairoa at Irvines
Example 4 – Double peak flood, threshold not reached
50.0
45.0
40.0
35.0
30.0
25.0
20.0
15.0
10.0
6 cumecs
5.0
0.0
12-Mar-2007
14-Mar-2007
16-Mar-2007
12-Mar-2007 00:00:00 to 21-Mar-2007 00:00:00
Flow (m3/sec) at HY Wairoa at Irvines
MovingMean7Day at HY Wairoa at Irvines
18-Mar-2007
20-Mar-2007
Example 5 - Large flood, threshold reached within 5 hours
50.0
45.0
40.0
35.0
30.0
25.0
20.0
15.0
10.0
6 cumecs
5.0
0.0
16-Dec-2010
18-Dec-2010
20-Dec-2010
16-Dec-2010 00:00:00 to 26-Dec-2010 00:00:00
Flow (m3/sec) at HY Wairoa at Irvines
MovingMean7Day at HY Wairoa at Irvines
22-Dec-2010
24-Dec-2010