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. Thirty-three hectares were harvested, 1% of the planted forest catchment area, along with roading and landing upgrades. Total harvested area since 1993 is 86% of the planted forest area, and 30% of the total catchment area.
- Mean annual baseflow (excludes flood data) water clarity in 2017 ranged from 4.8m at Te Pohatu at the top of the catchment, to 3.4m at the bottom of the catchment at SH1.u00a0
- Annual rainfall in the Waimarino catchment in 2017 (1650 mm) was the third highest on record and higher than the long-term average (1432 mm). The mean daily flow in 2016 (4198 l s-1) was at the high end of the range recorded over the monitoring period and above the long-term average (3358 mm) and the number of flood days (47) was the highest on record. Two flood events were sampled in 2017.
- Suspended sediment (SS) concentrations in the Waimarino River were < 5g/m3 for the majority of time, particularly at the two upper sites, with high concentrations usually associated with high flow events. SS concentrations in 2017 were low, similar to the previous three years. SS concentrations were positively associated with flow and negatively associated with water clarity at all three sites. Mean annual baseflow SS concentrations at S. H. 1 were significantly higher than the two upstream sites.
- Both total nitrogen (TN) and mean annual baseflow TN concentrations had been steadily increasing at all 3 sites since 2004, peaking at around 2012-2013 but showed a marked decline from 2014 to 2017. Mean annual baseflow TN concentrations show a similar trend, indicating that the number of floods sampled each year isnu2019t a major contributor to this trend. This trend isnu2019t 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 2.4-3.9 kg N ha-1 yr-1 in 2017 and were slightly higher than in 2016, influenced by the higher TN concentrations during the winter months.
- TN concentrations were positively associated with flow and SS, with the TOXN (total oxidised nitrogen) component of TN (averaging 42%-53% of TN) strongly influencing this result.
- Total phosphorus (TP) concentrations in the Waimarino in 2017 were similar to previous years, with the exception of a high TP concentrations at Te Pohatu in January 2017. We could not identify any obvious explanation for this results. TP concentrations showed a stronger positive relationship with SS than with flow. Mean annual baseflow TP concentrations at S. H. 1 were significantly higher than at the two upstream sites.
- TN:TP ratios in 2017 were within the N-limiting range at the upper (Te Pohatu) and lower (S.H.1) sites and within the ranged for balanced algal growth at Kepa Road.
- Harvesting began in the Mangakowhitiwhiti catchment in 1998 and continued through to 2005 with re-planting through to 2006. There have been no harvesting, roading or planting activities in the Mangakowhitiwhiti catchment from 2007 to 2016. In 2017, 11ha was harvested along with associated roading and landing upgrades.
- The total rainfall in 2017 (2229 mm) and number of rain days (183) were above the long-term average, whereas the daily maximum rainfall of 63 mm was below the long-term average. No flood events were sampled in the Mangakowhitiwhiti in 2017 and flows were within the normal range at both Kiko Road (upper catchment site) and Te Waaka Eruera Road (lower catchment site).
- Water clarity in 2017 was within the normal range for this catchment and no strong temporal changes were evident.
- Historical patterns in TN and mean annual TN baseflow concentrations in the Mangakowhitiwhiti were similar to those observed in the Waimarino catchment. TN concentrations at Kiko Road are significantly lower than at the two downstream sites. TN yields from Kiko Road and Te Waaka Eruera Road in 2017 were within the range of previous years (1.22 and 0.99 kg N ha-1 yr-1 respectively).
- TP concentrations were within normal range in 2017, with mean annual baseflow TP concentrations significantly lower at Kiko Road compared with the two lower sites. TP shows a positive increase with both 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.
- The interrelationships between water quality variables and flow in the Mangakowhitiwhiti catchment, in most instances, were similar to the Waimarino and similar to the Waimarino no strong temporal changes in water quality have been identified.
- 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 and have subsequently declined through to 2016, (Figures 15 & 16) most likely in response to the increasing demand for water and evapotranspiration rates from the growing tree crop and understory. Flow rates were particularly low from 2013 to 2016, a reflection of the low rainfall in both those years. Flow rates increased in 2017, a year of high rainfall.
- SS concentrations were more variable and higher in the pre-harvest, harvest and immediate post-harvest period until around 2002 (Figure 17). SS concentrations have been low and stable since then with high concentrations usually associated with high flow events. Mean annual SS concentrations show a similar trend (Figure 18). The higher rainfall and flows in 2017 showed no obvious impact on SS concentrations which were within normal range for the post-harvest period (Figures 17 & 18).
- TN concentrations in the Waimarino sub-catchment increased following harvesting operations until around 2003. Since then TN concentrations have declined 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 mobile TOXN component of TN leaching from the catchment which rapidly declined over 2002 and 2003 as the vegetation recovered. Both TN and TOXN were elevated in 2017 particularly during the winter and early spring period, most likely in response to the high rainfall, but were still within the post-harvest range of concentrations. Both TN and TOXN were positively correlated with flow whereas TKN was positively correlated with SS concentrations.
- The increase and subsequent decrease in mean annual TN yields from the sub-catchment coincided with the harvest and post-harvest periods. TN yields had returned to pre-harvest levels by 2003, and have remained at similar levels since (< 0.5 kg N ha-1 yr-1), although they were slightly elevated in 2017.
- 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 has shown little variation over the monitoring period. TP concentrations were positively correlated with SS concentrations.
- 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 2016. In 2017, TN:TP ratios varied between N-limiting and the range for balanced algal growth.