Assessing the Effects
NZFM carry out a regular water quality monitoring programme within Lake Taupo Forest. The main objective of the programme is to assess the effect of forest harvesting on water quality in the Waimarino River and the Mangakowhitiwhiti Stream. The potential impacts of harvesting are determined by measuring suspended sediment, nitrogen and phosphorus concentrations and water clarity.
Water quality assessments are undertaken at 3 sites along each watercourse. The location of these sites were carefully chosen in each catchment to ensure that water quality measurements were taken from upstream, within and downstream of the plantation forest boundary.
- Harvesting commenced in 1993 in the Waimarino catchment, finishing in 2006 (total area 2454 ha). Re-planting ceased in 2007 (total area 2338 ha) and an additional 241 hectares of pasture land was planted in 2013. After a ten-year hiatus, harvesting recommenced in the Waimarino catchment in 2017 and replanting in 2018, Five hectares was harvested in 2018 and 33 ha was replanted along with roading and landing upgrades/ establishment. The total harvested area since 1993 remains at 86% of the planted forest area, and 30% of the total catchment area.
- Annual rainfall in the Waimarino catchment in 2018 (1427 mm) was below the long-term average (1432 mm) and the number of rain days (174) was the highest on record. The mean daily flow in 2018 (3744 I s-1) was slightly above the long-term average (3358 mm). No flood events were sampled in 2018. No temporal trends in rainfall and flow were identified.
- Annual baseflow (excludes flood data) water clarity in 2018 averaged 4.2m at Te Pohatu at the top of the catchment, 3.5 m at Kepa Road part way down the catchment and 3.3 m at the bottom of the catchment at S.H.1. There is a significant decline in baseflow water clarity down the main stem of the Waimarino River. Water clarity (all data including floods) significantly declined with increasing flow at all three sites.
- Suspended sediment (SS) concentrations in the Waimarino River in 2018 were <5g/m³ for most sampling events, similar to the previous four years. Higher concentrations were associated with high rainfall and flow events. SS concentrations were positively associated with flow and negatively associated with water clarity at all three sites and mean annual baseflow SS concentrations at S.H.1 were significantly higher than the two upstream sites.
Over the time period of this project, total nitrogen (TN) declined at all three sites from 1994 to 2003, with a subsequent increase peaking at around 2013, followed by a decline that has continued into 2018. Mean annual baseflow TN concentrations show a similar trend, indicating that the number of floods sampled each year isn’t a major contributor to this trend. This trend isn’t related to harvesting operations which ceased in 2007 or other land use related activities as the indigenous forest site is experiencing the same trends, indicating that other factors are influencing TN concentrations in this catchment. Mean annual TN yields range 1.4-2.1 kg N ha-1 yr-1 in 2018.
TN concentrations were positively associated with flow and SS, with the total oxidised nitrogen (TOXN) component of TN strongly influencing this result.
Total phosphorus (TP) concentrations in the Waimarino in 2018 were low, ≤0.3 g/m³. Mean annual baseflow concentrations at S. H. 1 were significantly higher than at the two upstream sites. TP concentrations show a significant positive relationship with flow and a stronger positive relationship with SS.
TN:TP ratios at all three sites in 2018 were some of the lowest on record and were within the N-limiting range for algal growth.
Harvesting began in the Mangakowhitiwhiti catchment in 1998 and continued through to 2005 with re-planting through to 2006. There were no harvesting, roading or planting activities in the Mangakowhitiwhiti catchment between 2007 and 2016. Harvesting recommenced in 2017 and in 2018 83 ha were harvested and 7 ha replanted along with the establishment and upgrade of roads and landings.
The total rainfall in 2018 (1663mm) was below the long-term average (1848 mm) for the catchment. Flows, and mean annual baseflow, were within the normal range at both Kiko Road (upper catchment site) and Te Waaka Eruera Road (lower catchment site). One flood event was sampled in 2018.
Water clarity in 2018 averaged 4.3 m at Kiko Road at the top of the catchment, 2.5 m at Te Kaka Road part way down the catchment and 1.8 m at the bottom of the catchment at Te Waaka Eruera Road. There is a significant decline in baseflow water clarity down the Mangakowhitiwhiti Stream. Water clarity (all data including floods) significantly declined with increasing flow at all three sites.
SS concentrations in the Mangakowhitiwhiti in 2018 were <8 g/m³ except in the one flood event that occurred during the year. SS concentrations increased significantly with increasing flow rates. Mean annual baseflow SS concentrations in 2018 were within the range of previous years and showed a significant increase down the main stem of the Mangakowhitiwhiti.
- Temporal patterns in TN and mean annual TN baseflow concentrations in the Mangakowhitiwhiti, before and after harvest, were similar to those observed in the Waimarino catchment. TN concentrations were positively correlated with increasing flow and SS concentrations. Mean annual baseflow TN concentrations were at the low end of the range in 2018 and the lowest on record at Kiko Road. Baseflow TN concentrations at Kiko Road were significantly lower than at the two downstream sites. TN yields from Kiko Road and Te Waaka Eruera Road in 2018 were 1.5 and 1.1 kgN ha-1 yr-1
- TP concentrations in 2018 were ≤0.3 g/m³ and the mean annual baseflow TP concentrations were significantly lower at Kiko Road compared with the two lower sites. TP concentrations showed a positive and significant increase with increasing flows and SS concentrations.
- For most of the monitoring period (including 2017), TN:TP values at the three sites have predominantly been in the balanced range for algal growth although from 2008 to 2013 TN:TP values have been predominantly in the P limiting range at the two lower sites, most likely a response to increasing TN concentrations during those years. TN:TP ratios in 2018 were low, in response to TN concentrations and were in the N-limiting range at all three sites.
- The Waimarino sub-catchment was harvested in 1997 and 1998 and re-planted in 1998. Flow rates in this sub-catchment are generally two orders of magnitude smaller than in the main Waimarino catchment. Flow rates and mean annual flow rates were higher during the pre-harvest, harvest and immediate post-harvest period. Flows decline from around 2003 to 2016 but were elevated during 2017 due to high rainfall that year and have persisted into 2018. However, the long –term temporal trend still shows an overall significant decline in flow in the sub-catchment as the tree crop continues to grow.
- SS concentrations were more variable and higher in the pre-harvest, harvest and immediate post-harvest period until around 2002. SS concentrations have been low and stable since then and averaged 5.8 g/m³ in 2018. High SS concentrations were usually associated with high flow events. SS concentrations show small but significant increase with increase flow.
- TN concentrations in the Waimarino sub-catchment increased following harvesting operations until around 2003. Since then TN concentrations have declined through to 2018 (averaging 0.11 g/m³) although they are generally higher and more variable than in the pre-harvest period. The post-harvest increase in TN was almost entirely due to the more TOXN component of TN leaching from the catchment which rapidly declined over 2002 and 2003 as the vegetation recovered. Both TN and TOXN were positively correlated with flow whereas TKN was positively correlated with SS concentrations.
- Mean annual TN yields from the sub-catchment followed a similar temporal pattern to TN and averaged 0.2 kgN /ha/yr in 2018.
- The TP response to harvesting was muted, most likely reflecting the adsorption of phosphorus onto the allophanic fractions of volcanic soils in the area and TP concentrations have shown little variation over the monitoring period, averaging 0.2 g/m³ in 2018.TP concentrations were positively correlated with SS concentrations but showed no significant response to flow.
- TN:TP ratios in the sub-catchment were predominantly N-limiting prior to harvest. During the harvest and post-harvest period the increase in TN resulted in TN:TP ratios that were predominantly P-limiting, but TN:TP ratios subsequently returned to the N-limiting range around mid-2004 and have remained in that range, most of the time, through to 2018.