Mapping Gullies for Land Stabilisation

Gully erosion accounts for a large proportion of the sediment exiting from rivers draining the three major catchments in the Gisborne region, which together account for about 60% of the North Island’s total suspended sediment yield. Afforestation of farm land previously in pasture has reduced gully erosion substantially and thus reduced the sediment load in these rivers. Landcare Research has mapped gully distribution at different periods in time, developed models for predicting erosion rates and gully stabilisation following the reforestation of gullies and applied these to the Waipaoa, Waiapu and Uawa catchments to assess the effectiveness of soil conservation measures. Its analyses indicate that historic afforestation in Waipaoa catchment has reduced sediment yields by 30%, but more gullies throughout this region need to be reforested otherwise sediment yield could increase substantially.

Three catchments in the Gisborne region account for about 60% of the total sediment flowing out to sea from all North Island rivers combined. The Waiapu River alone carries around 35 million tonnes of suspended sediment annually – the highest yield of any NZ river – and that amount is increasing. The other two catchments, the Waipaoa and the Uawa, also produce huge amounts.

A major part of the problem is that gullies contribute the equivalent of about half of the suspended sediment load exiting the major river systems and about 800 gullies are still active and require treatment to stabilise them. Because the gullies are linked to stream channels, they leak sediment all year round and even with small rainfall events the water runs dirty. The actively eroding areas comprise around only 1 – 2% of total hill country but the area affected is about 100 square kilometres and its productivity has been substantially reduced.

The problem started over a century ago when huge tracts of the region were cleared for pastoral farming. Erosion, deepening and expansion of existing gullies ensued, along with the creation of new ones. The situation did not start to abate until the 1960’s when a programme of reforestation was started. Since then around 135,000 ha of exotic forest have been established and planted gullies have been successfully stabilised. However, the area affected by gully erosion in the wider Gisborne region has actually increased by a quarter because in the past 40 years gullies on unforested farmland have increased in size and new gullies have formed.

Dr Mike Marden, a scientist with Landcare Research in Gisborne, has studied the effects of forest growth on gully formation and stability. He says that maps of the distribution of gullies within Mangatu Forest (the earliest forest to be planted in this region for erosion control) as at 1939, 1960, 1970, and 1988 show the expansion of gully erosion under a pastoral regime (1939-1960) followed by a reduction in gullies as the area was progressively reforested until 1988 when all but the largest gullies had stabilised.

“When we say that gullies have stabilised we make the assumption that once a gully has been planted and at some later date it can no longer be seen on aerial photography because the trees have provided a closed canopy, then the trees are deemed to have stabilised the ground underneath, thus the gully has been shut-down and is no longer producing sediment” says Dr Marden.

Dr Marden has also used aerial photography taken prior to 1960, when the NZ Forest Service first started planting trees, and compared them with photographs of the whole region taken in 1997.

He mapped gully distribution within the three major catchments – the Waipaoa, the Uawa, and Waiapu – and studied the change in gully numbers and area over that time period to determine the influence of forest planting on stabilising gullies that developed when these areas were farm land. As you might expect, the number and size of gullies decreased within the reforested area but across each of the three catchments as a whole it was a different story entirely.

“The sad fact is that if you took the composite gully area there is actually an increase between 1957 and 1997, so while there has been a decrease in the areas that have been reforested over that timeframe, a whole lot of new gullies were initiated mainly on farmland, and the end result was a larger composite gully area than before reforestation started,” says Dr Marden.

“The main cause of the increase in gully number and composite area is that many new gullies were initiated during major storms in the 60’s and cyclones in 1980 and 1982. And then Cyclone Bola in 1988 resulted in extensive erosion on steep hill country farmland. A spin-off following these events was a renewed interest in reforestation as a means of preventing further erosion in the future. The Government’s response to the extent of damage in the region following Cyclone Bola was to initiate the East Coast Forestry Project, and so with the monies available through this grant scheme, a lot more farmland was either planted in forest or retired and allowed to revert to indigenous forest.”

“Also at that time there was increasing interest in forestry as an investment and that really drove the conversion of a lot of pastoral land that had become marginal. There was a big flurry of activity in the early 1990’s and a lot of uneconomic farms were snapped up and put into trees.”

“There is no doubt that if the East Coast Forestry Project had not been put in place, there would be many more gullies currently producing sediment.”

The forestry programme is ongoing and there is still money available for land owners who want to reinvest in forestry up until 2020. Although investment has waned over the last 7 years it is picking up again, possibly because of the interest in carbon farming (sequestration of atmospheric carbon into permanent forest trees).

Landcare Research has developed models for predicting the rate at which untreated gullies increase in size and the sediment generated from them, also the rate that sediment generation declines following reforestation and has applied these models to all the remaining gullies within the Waipaoa and Waiapu catchments. It indicates that afforestation has reduced sediment yield by 17% in the Waiapu catchment and 33% the Waipaoa catchment.

Further, it indicates that the most effective way to reduce future sediment yield from gullies is to reforest all remaining gullies (and their watersheds) irrespective of size. If this were done for all existing and every newly initiated gully in the Waiapu catchment by 2020, then the sediment yield is predicted to halve by 2050 from 22 million tonnes to 11 million tonnes per year. However, if there is no more afforestation in the Waiapu catchment then the sediment yield will more than double by 2050.

The following shows one gully photographed at three different periods of time, the first in pasture, the second soon after it had been planted and the third one at the end of the rotation with mature trees, so you can see the same feature in its raw state, stabilising and then fully stabilised.

Mike explains. “The first is dated 1961 and shows a raw bedrock gully exposed and surrounded by pasture and all round the sides you can see the cracked hillsides falling into the gully. When you take away the forest you increase the amount of rainfall that becomes run-off that concentrates into a single channel and just slices through the brittle bedrock that underlies this area. The second dated 1972 shows relatively young trees, and you can see the extent of the plantings on a lot of the watershed slope areas around the central gully but they had very little soil medium in which to plant within the rawest part of the gully, so you can see the young trees established healthy and growing around the perimeter. The third photograph from 2004 shows the completely closed canopy but you can see that they had another go at replanting the raw area using a different species and so have been able to fully stabilise the gully.”

“The area is due for harvesting sometime soon and that will be interesting when they take the trees away. In other areas where they have harvested trees the gullies become very visible again but there hasn’t been any indication that the rate of sediment generation from them has increased. One factor is that there haven’t been any significant storms since Cyclone Bola to coincide with the harvesting of these gullies but if that did happen there would probably be an increase in activity in these features. So far they (forest owners) have got away with it and they have been able to replant them, and those plantings have re-stabilised that feature before any storm has shown up.”

“Of course there is only one small part of the catchment that is laid bare at any one time and in any case gullies occupy only about 2% of the hill country area in each of the major catchments and less than 1% of hill country across the entire East Coast Region.”