Ngaruroro WCO Hearing Part 2 Lower Catchment Hydrology Introduction - PowerPoint PPT Presentation

Ngaruroro WCO Hearing Part 2 Lower Catchment Hydrology

Introduction • In my evidence to the first Hearing, I provided information on the hydrology of the Ngaruroro River at and upstream of the Whanawhana River flow recording site. • This evidence included flow statistics for the recorder sites in the upper catchment along with analysis of trends in mean, high and low flows. • These trends in mean, high and low flow statistics are generally applicable to the whole catchment since they are calculated using natural flows. • Flow recorders downstream of Whanawhana are affected by abstractions and natural losses to groundwater. 2

Physical Description • The catchment area of the lower Ngaruroro River downstream of Whanawhana is around 900km 2 compared with the almost 1100km 2 for the upper catchment. • Compared to the often mountainous terrain of the upper catchment, the lower catchment terrain is generally rolling farmland and much less steep than the upper catchment. • The Clive River catchment is almost 500km 2 in area and includes the large flat area of the Heretaunga Plains. 3

Ngaruroro Catchment 4

Lower Ngaruroro Catchment 5

Clive River Catchment 6

Climate • The climate of the Ngaruroro catchment was addressed in my evidence to the first Hearing so only a brief summary is provided here. • Hawkes Bay is one of the sunniest, least windy regions in New Zealand. Most of the rainfall occurs from the easterly quarter and annual totals range from 800mm on the Heretaunga Plans to in excess of 2000mm in the Ngaruroro River headwaters. 7

Median Annual Rainfall 8

The Lower Ngaruroro River • The Ngaruroro River riverbed downstream of the Whanawhana recorder site changes from a rocky/bouldery bed to one of mostly gravels, sands and silts. • There are a number of tributaries of the Ngaruroro River downstream of Whanawhana but very few are measured for the full range of flows. In this evidence I have considered only the main stem of the river and one tributary to show the difference in water yields between the main river and its downstream tributaries. • There are two recorder sites with full flow records on the main river downstream of Whanawhana, one at Fernhill and the other at Chesterhope Bridge, about 11km downstream of the Fernhill site. 9

Lower Ngaruroro Recorder Sites 10

Flow Recorders • Discussion of the Whanawhana site can be found in my evidence to the first Hearing • Fernhill is about 45km downstream of Whanawhana and about 17km upstream of the Waitangi estuary. HBRC provided records to me starting in February 2005. Jeff Smith, hydrologist for HBRC, has used additional data beginning in 1998 that was not provided to me. In any event, any differences between his and my analyses are immaterial for the purposes of this application and for a general understanding of the catchment hydrology • The Fernhill site has been installed since 1952. There are many rating changes because of the nature of the riverbed. The Fernhill site is that used by HBRC for implementing low flow restrictions when needed. • The other mainstem site is Ngaruroro at Chesterhope, about 11km downstream from Fernhill. NIWA operate this site. There is a more stable riverbed at this site probably because it is in a man-made river channel. Good usable record for this site began in October 1978, so 40 years of record is available. 11

Flow Recorders - Continued • The other recorder site included here is the Maraekakaho tributary (about 100km 2 in area) and it is included for comparison purposes in terms of water yield. It joins the Ngaruroro about 30km downstream of Whanawhana. • In order to compare the 3 mainstem sites fairly, the record lengths need to be the same. The following table provides that comparison for the period 2 February 2005 to November 2018. This is the only period of concurrent record for all three sites. 12

Flow statistics for the Ngaruroro River at three sites for 2 February 2005 to October 2018 (l/s) Mean Median MALF Specific Discharge Max Min Mean Fre3 (l/s/km 2 ) Annual Flood Inst. 7 -Day Chester 44,324 24904 4380 4850 22.2 1529400 2015 1111600 7.1 Fernhill 32,150 21330 4150 4210 16.7 1509000 1230 1194805 5.8 Whana 34,170 22140 7720 8270 31.3 720470 4920 477490 6.8 Maraek 805 305 116 140 8.0 298300 13 101600 5.8 • Note that Fernhill data is often inconsistent with the Chesterhope data 11.5km downstream. This could be due to flow gains in the reach and/or data inaccuracy. • Mean and median flows increase with distance downstream between Whanawhana and Chesterhope. • Minimum recorded flows, and instantaneous and 7 day MALFs decrease downstream, likely due to natural losses to groundwater and abstractions. 13

Maximum Measured Flows Date Ngaruroro River Max. Flows (l/s) Chesterhope Whanawhana 9 March 1988 2115000 1026400 28 December 1980 2081800 615300 12 June 2018 1529000 682200 1 February 2010 1436000 720500 30 July 2008 1428900 589500 14

Low Flows and Flow Losses • The Fernhill and Chesterhope sites are affected by both natural losses and abstractions. Most of the natural losses occur in the reach from the Ngaruroro/Maraekakaho confluence to Fernhill with most occurring between Roys Hill and Fernhill. • HBRC measurements of median loss in this reach was 4250 l/s in the major loss reach. In the reach upstream of Roys Hill, a possible loss of 150 l/s, and in the reach from Fernhill 3 km downstream, a loss of 60 l/s. 15

Major lo losing reaches in in th the Lo Lower Ngaruroro River (b (based on Fig igure 3, , page 5 of f HBRC Memo dated 13 March 2017) 16

Clive River Catchment • Lowland catchment with maximum elevation of 490m. • The section of river known as the Clive River is the former outlet for the Ngaruroro River. That river now discharges through a man — made channel into the Waitangi Estuary. • The Clive River tributaries of Raupare and Karamu Streams were tributaries of the Ngaruroro. In 1969, the Ngaruroro was diverted through the man-made channel and these tributaries now flow out of the old Ngaruroro channel, known as the Clive River. 17

Clive River Catchment Area 18

Clive River Catchment Flows • The Clive River catchment contains most of the Heretaunga Plains and Heretaunga Aquifer System. The aquifer is largely recharged from natural losses from the Ngaruroro River. • HBRC notes that losses from the Ngaruroro have increased and spring gains in the tributaries of the Clive River close to the Ngaruroro catchment have declined. Increased groundwater pumping has caused reduced streamflow, especially in summer. • HBRC estimate that groundwater pumping is causing a depletion of about 1000 l/s from the Ngaruroro during the driest condtions. • Flows in the Clive River are mainly a combination of water originating from the upland parts of the catchment and groundwater springs and seepage from the Heretaunga Aquifer with water originating from the Ngaruroro River. • There are no flow records available for the Clive River. Estimates of flow in the Clive River using measured tributary flows are MALF approximately 1370 l/s and median flow approximately 2400 l/s. 19

Allocation • Both the Ngaruroro River (including its Maraekakaho and Tutaekuri- Waimate tributaries) and the main streams contributing to the Clive River are fully allocated. • HBRC has stated that groundwater takes from the Heretaunga Aquifer are at the limit of what is environmentally acceptable. • From the HBRC consents database, there are approximately 72 surface water abstraction consents and more than 1200 groundwater abstraction consents within the greater Ngaruroro and Clive River catchments. 20