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Spatial and temporal distribution of rainfall erosivity in New Zealand.

Introduction

Rainfall and its kinetic energy expressed by rainfall erosivity is the main driver of soil-erosion processes by water. These processes are associated with detachment of soil particles, runoff generation and triggering of mass movements (Nyssen et al. 2005). The rainfall-runoff erosivity factor (R) of the Revised Universal Soil Loss Equation (RUSLE, Renard et al. 1997) is one of the most widely used parameters describing rainfall erosivity. This factor includes the cumulative effects of the many moderate-sized storms as well as the effects of the occasional severe ones; R quantifies the effect of raindrop impact and reflects the amount and rate of runoff associated with the rain.

The original method to calculate erosivity values (R-factor) for a storm event requires pluviographic records (Wischmeier and Smith 1978) with temporal distribution of both the amount and intensity of precipitation. When this information is lacking, other approaches using daily input (e.g. Bagarello and D'Asaro 1994; Yu and Rosewell 1996; Davison et al. 2005) are appropriate because long-term series of daily rainfall are available at most sites. Monthly and/or annual values of rainfall erosivity can also be assessed by using annual and monthly rainfall averages (Renard and Freimund 1994; Gabriels 2006). The spatial distribution of erosivity values can be displayed in so-called isoerodent maps, showing lines of equal erosivity. Such maps exist for many countries, for example the USA (Wischmeier and Smith 1978), Brazil (da Silva 2004), Switzerland (Meusburger et al. 2012), Chile (Bonilla and Vidal 2011), Spain (Angulo-Martinez and Begueria 2009), Honduras (Mikhailova et al. 1997), parts of Germany (Schwertmann et al. 1987), parts of Australia (e.g. Sheridan and Rosewell 2003), and Austria (Klik and Konecny 2013).

Nationwide soil-erosion assessments for New Zealand have been carried out by Griffiths and Glasby (1985) and Hicks et al. (1996). Both approaches used mean annual rainfall as a driving force of erosion. Dymond et al. (2010) improved the model proposed by Hicks et al. (1996) by incorporating land cover, but their approach still used mean annual precipitation, although recognising that rainfall intensity is likely to be a stronger driver of erosion than is annual rainfall. There remains a need, therefore, to provide an assessment of rainfall erosivity in New Zealand to improve understanding of nationwide patterns of soil erosion.

The objectives of this study were (i) to determine the R-factor for different climatic regions in New Zealand, in order to create a spatially distributed rainfall erosivity map; and (n) to analyse the temporal distribution of erosive rainstorms. We used rainfall breakpoint data from 35 rain gauges distributed over the North and South Islands. Based on these data and with the addition of 597 other gauging stations with 30-year mean annual precipitation amounts, we derived a rainfall erosivity map for New Zealand.

Materials and methods

Study area

New Zealand, with an area of ~269 600 [km.sup.2], is geologically young and located in a highly active tectonic setting at the boundary between the Indo-Australian and Pacific plates, which generates uplift rates up to 10-20 mm [year.sup.-1] in parts of the Southern Alps (Coates 2002). The country is characterised by a diversity of landscapes underlain largely by relatively soft and/or erodible lithologies. Volcanic activity from the Taupo Volcanic Zone in the North Island contributes tephra to land surfaces downwind and in the vicinity of eruptive centres. Nearly the whole country (with the possible exception of the Far North) is susceptible to frequent earthquakes. Much of the country is hilly and it could be classified as steepland terrain (sensu Gomez et al. 2010) dissected by short, steep rivers with relatively small floodplains. Very high rainfall (up to 15 000mm annually projected at the divide of the Southern Alps) and strong prevailing westerly winds dominate New Zealand's climate. In general, the climate can be described as temperate with sharp regional contrasts. A strong oceanic influence results in rapidly changing and variable weather patterns. High annual rainfall, steep slopes, small catchments and powerful rivers, combined with active tectonics and erodible rock types, are the basis for a high rate of natural and accelerated erosion (Jakobsson and Dragun 1991). Of the 209 Mt of suspended sediment yielded annually to the ocean from New Zealand, the east coast contributes ~55 Mt (Hicks and Shankar 2003), which equates to -0.3% of the total global suspended sediment input to the ocean, derived from ~0.003% of the total land area of the Earth (Marden et al. 2008). The alignment of two relatively narrow islands with steep and high relief perpendicular to the prevailing westerly winds coming off the Southern Ocean and Tasman Sea generates very high precipitation gradients between west and east coasts of both North and South Islands.

The distribution of rainfall is mainly controlled by mountain features. Highest rainfalls occur on the west coast of both islands, where the mountains are exposed to prevailing westerly and northwesterly winds from depressions approaching from the southwest. The 30-year mean annual rainfall (1981-2010) of 597 rain gauges over all New Zealand ranges between 460 mm in a small area of Central Otago, in the south-west of the South Island, and 6715 mm in the Southern Alps, which span the west coast of the South Island (Fig. 1). The average precipitation for the whole country is high, but for the greater part lies between 600 and 1500 mm. The only areas with average rainfalls <600 mm are found in the South Island to the east of the main ranges, and include most of Central and North Otago, and south Canterbury. In the North Island, the driest areas are central and southern Hawkes Bay, Wairarapa, and Manawatu, where the average rainfall is 700-1000 mm a year. Of the remainder, much valuable farmland, chiefly in northern Taranaki and Northland, has >1500 mm. Over a considerable area of both islands, rainfall exceeds 2500 mm [year.sup.-1].

Areas that are exposed to the west and south-west experience much showery weather, and rain falls on roughly half the days of the year. Over most of the North Island, there are at least 130 rain days per year, except to the east of the ranges where in places there are <110 rain days. The areas of the South Island with annual rainfall <600 mm generally have ~80 rain days per year. In the far south, the frequency of rain increases sharply, rain days exceeding 200 per year in Stewart Island and Fjordland.

New Zealand Institute of Water and Atmospheric Research (NIWA) divided the country into nine climate zones, with four zones in the North Island and five in the South Island (Fig. 1).

Warm humid summers and mild winters are typical for Northern New Zealand (Kaitaia, Whangarei, Auckland, Tauranga). Tropical storms from the east or north-east are likely to occur in summer and autumn. The mean annual rainfall for this region is 1100-1300 mm. Central North Island (Hamilton, Taupo, Rotorua) is sheltered by mountains to the south and east, which leads to less wind then in many other parts of New Zealand. The mean annual rainfall strongly depends on the altitude, with 1000-1300 mm for Hamilton, Rotorua and Taupo, and up to 3000 mm at Mount Ruapehu/Tongariro National Park. South West North Island (New Plymouth, Wanganui, Palmerston North, Wellington) is very exposed to the weather fronts coming from the Tasman Sea and is therefore a very windy region. The mean annual rainfall in this region is ~1000-1200 mm with a peak in New Plymouth (1500 mm) and Mount Taranaki (up to 8000 mm). Eastern North Island (Gisborne, Napier, Masterton) is dominated by winds from the north-east, with annual precipitation amounts of 770-1000 mm.

Northern South Island (Takaka, Nelson, Blenheim) is the sunniest region of New Zealand. Mean annual precipitation ranges between 700 and 1000 mm, with peaks of 2000 mm occurring in Golden Bay. Precipitation in Western South Island (Westport, Hokitika, Milford Sound) varies greatly and depends on the exposure to the Tasman. Annual values can reach up to 10 000 mm in the Southern Alps. The highest rainfall occurs in a narrow band where the prevailing westerly winds force moist air from the Tasman Sea over the mountains (Henderson and Thompson 1999). In Eastern South Island (Kaikoura, Christchurch, Timaru), annual precipitation is low, with values between 600 and 700 m. Long dry spells in summer are common. The Inland South Island (Lake Tekapo, Alexandra, Manapouri, Queenstown) shows a high gradient from east to west. Alexandra is the driest place in whole of New Zealand, with average annual precipitation ~350 mm. whereas values increase to 3000 mm in Manapouri. Southern New Zealand (Dunedin, Invercargill) is not sheltered in any direction and is therefore influenced by weather fronts from the south-east. Precipitation ranges from ~700 mm [year.sup.-1] in Dunedin to 1200 mm [year.sup.-1] in Invercargill.

Database and calculation of rainfall erosivity

We used high-resolution rainfall data (10-min intervals) from 35 gauging stations for the calculation of the rainfall erosivity. Fifteen stations were located in the North Island and 20 stations in the South Island (Fig. 2).

For the calculation of the R-factor, a period of data collection at least 22 years is recommended (Renard et al. 1997). Length of available data records from N1WA ranged only from 4 to 16 years, with an average of 11 years. In absence of longer records with high temporal resolution, the R-factor was analysed from these (shorter) data records. A study by Oliveira et al. (2013) showed that good R-factor results could also be obtained from shorter data series.

The R-factor takes into account the kinetic energy (KE) and the maximum 30-min rainfall intensity ([I.sub.30]) for all rainstorms occurring during 1 year. Two consecutive rainstorms are distinguished when a dry period of 6 h separates these storms (Renard et al. 1997). We used a threshold of 12.5 mm or 12.5 mm [h.sup.-1] to distinguish between non-erosive and erosive storms. From the 35 stations, 398 station-years with 13 612 rainstorms fulfilled these requirements and were analysed. The kinetic energy for each rainfall event was calculated based on the approach of Brown and Foster (1987), by using the breakpoint precipitation data for each storm. The rainfall energy per unit depth for each time increment ([e.sub.r]) was determined using the following equation:

[e.sub.r] = 0.29(1 - 0.72 [exp.sup.-0.051]) (1)

Total storm kinetic energy for a single rain event ([E.sub.i]) is the product of the rainfall energy per unit depth for each time increment ([e.sub.r]) and the total depth during that increment ([DELTA][V.sub.r]), summed up over the entire rain event:

[E.sub.i] = [o.summation over (r=1)] [e.sub.r][DELTA][V.sub.r] (2)

with o the number of increments for the particular rainstorm.

We obtained the average annual rainfall and runoff erosivity factor (R, MJ mm [ha.sup.-1] [h.sup.-1] [year.sup.-1]) by summing the products of the total kinetic energy E and the maximum [I.sub.30] for all rainstorms during n number of years:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

For determining E and [I.sub.30], the Rainfall Intensity Summarisation Tool (RIST; USDA-ARS 2010) was used, which calculates the values of precipitation, duration, maximum 30-min intensity, rainfall kinetic energy E and [EI.sub.30] for each storm. To investigate the intra-annual distribution of the R- factor, data were separated into spring (September-November), summer (December-February), autumn (March-May) and winter (June-August) periods. In addition, for each station, the average 30-min rainfall intensity, the average R for a single storm event and the average number of erosive storms per year were calculated.

The spatial distribution of the calculated R-factors from the 35 rain gauges was compared with the nine climatic regions (Fig. 1). We found that for rainfall erosivity, these nine regions could be merged into three erosivity regions. For each of these regions, the relationship between R-factor and annual precipitation could be best described by a linear or nonlinear regression function. From NIWA, we obtained long-term mean annual precipitation data (1981-2010) for 597 rain gauges spatially distributed all over New Zealand. These stations were divided according to their location into the three erosivity regions. Corresponding R-factors were then calculated by applying the appropriate region-specific function. Panagos et al. (2014) used a similar approach to propagate calculated R-factor results from four rain gauges with breakpoint data to 24 stations with long-term average monthly and annual precipitation in Crete. Interpolation between the 597 data points was then done by universal kriging (Angulo-Martinez et al. 2009).

Results and discussion

Rainfall erosivity

For the investigated period, a mean annual precipitation of 1357 mm was obtained from the 15 observed stations in the North Island (Table 1). Rainfall distribution is relatively even in spring, summer and autumn, with 22-24% of annual rainfall occurring during each of these seasons (Fig. 3). Highest rainfall was observed in winter. In the South Island, the mean annual rainfall of the 20 analysed stations amounted to 2040 mm (Table 1), therefore higher than in the North Island. Temporal variation throughout the year is very low. The observed seasonal range of precipitation was between 24% (winter) and 26% (summer). The mean values of the 339 stations in the North Island and the 258 stations in the South Island showed similar results (Table 2), although there is a high spatial variability within and between both islands. Additionally, the mean values are affected by the spatial distribution of the rain-gauge network, with higher density of stations in areas with lower precipitation.

A comparison between long-term average annual precipitation (1981-2010) and the mean values derived from the high-resolution data shows a very good linear regression for the investigated stations ([R.sup.2] = 0.96), although the 30-year values are ~6% higher (Fig. 4).

The range of rainfall erosivity R varies greatly, and is higher in the South Island. In the North Island, 196 station-years with 5941 erosive storms were evaluated. The results show that precipitation between 718 mm [year.sup.-1] (Napier) and 2714 mm [year.sup.-1] (Mount Ruapehu) delivered R-factors between 477 and 3592 MJ.mm [ha.sup.-1] [h.sup.-1] (Table 1). In the South Island, 202 station years with 7671 storms were evaluated. Corresponding mean annual rainfall of 429-4300 mm produces erosivity values of 230-10 895 MJ.mm [ha.sup.-1] [h.sup.-1] (Table 1).

We found out that the nine climatic regions derived by NIWA could be aggregated into four regions with similar erosivity pattern. This classification is based on the correlation between annual or seasonal precipitation and rainfall erosivity. The annual rainfall as well as seasonal erosivity can be described by a power or linear function:

R = [aP.sup.b] (4)

or

R = aP + b (5)

where P is the annual or seasonal precipitation (spring, summer, autumn and winter), respectively, and a and b are constants. The type of function and the coefficients describing the regressions are displayed in Table 3. The determination coefficients [R.sup.2] of the developed relationships are high (Table 3).

Griffiths and Glasby (1985) used a national dataset of sediment discharge of 80 rivers to derive a rainfall factor of [P.sup.2.3] for both islands. In a more comprehensive study of 203 sites with sediment discharge data, they evaluated an erosivity of annual rainfall raised to the power of 1.7. Dymond et al. (2010) evaluated both erosivity approaches and found little difference for the accuracy of erosion model predictions. In this study, we calculate b values for annual rainfall erosivity of 1.13-1.53, depending on the region. Therefore, our exponents are smaller than those that were derived in previous studies.

Based on the data and the developed relationships, following regions can be distinguished (Table 3):

1. Region 1 : northern New Zealand

2. Region 2: remaining North Island including eastern, central and south-west North Island

3. Region 3: northern and western South Island

4. Region 4: eastern South Island, inland South Island, southern New Zealand and southernmost part of western South Island

In region I, northern New Zealand, the mean annual rainfall of the four stations during the investigation period is 1206 mm with amounts of 1037-1388 mm. Precipitation is more or less evenly distributed over the year with a slight winter peak (Table 2). Despite this winter peak of rainfall, the most erosive storms occur in autumn (Fig. 3). Each year, on average, 28 erosive events with a mean 30-min intensity of 12.8 mm [h.sup.-1] and a storm erosivity of 86 MJ.mm [ha.sup.-1] [h.sup.-1] are counted, which produce an R-factor of 2499 MJ.mm [ha.sup.-1] [h.sup.-1].

The remaining part of North Island (region 2) shows a great variation in precipitation and erosivity (Table 2). Mean precipitation and rainfall erosivity of the 11 stations are 1413 mm and 1942 MJ.mm [ha.sup.-1] [h.sup.-1], respectively. The distribution of rainfall and erosive rainstorms is similar to northern New Zealand, with a rainfall peak in winter but a storm peak in summer (Fig. 3).

About twice as many erosive storms occur in this region as in northern New Zealand, but storms have a lower storm erosivity (56 MJ.mm [ha.sup.-1] [h.sup.-1]) and rainfall intensity (10.0 mm [h.sup.-1]).

In region 3, including northern and western South Island (except the southernmost part), the highest rainfall and highest erosivity occur (Table 2). For the eight investigated rain gauges, mean precipitation of 2953 mm is registered with a high variability of 921-4300 mm. It is well known that huge spatial variations in precipitation are found near mountainous terrain. Rainfall is generally enhanced about and upslope of mountain barriers, with a rain-shadow region to the lee. The Southern Alps rise to >3000 m above sea level and exert a huge impact on average rainfall patterns (Sinclair et al. 1997). Because of the location within the southern hemisphere westerlies, a large rainfall gradient occurs across this topographic barrier (Chinn 1979). Mean R-factor for this region is calculated as 6498 MJ.mm [ha.sup.-1] [h.sup.-1] from values between 1448 MJ.mm [ha.sup.-1] [h.sup.-1] (Nelson) and 10 896 MJ.mm [ha.sup.-1] [h.sup.-1] (Franz Josef). About 51 erosive storms are registered annually with an average erosivity of 105.6 MJ.mm [ha.sup.-1] [h.sup.-1] each (Table 2). The [I.sub.30] is estimated to be 11.6 mm [h.sup.-1] and, therefore, is slightly smaller than in northern New Zealand but higher than elsewhere in New Zealand. The stations Arthur's Pass and Mount Cook deliver 30-min rainfall intensities of 9.7 and 9.9 mmh [h.sup.-1], which are smaller than the mean value of that region. This can be explained by the higher altitudes of these two stations (738 and 765 m above sea level). Parts of the precipitation from late fall to late spring may occur as snow with very low intensity, leading to small kinetic energy values during this period.

In region 4, eastern and inland South Island are combined with southern New Zealand and the southernmost part of western South Island. In this region, the annual precipitation varies greatly between 429 and 3407 mm, with an overall mean of 1431 mm (Table 2). In addition, the R-factors have a large range from 229 to 3757 MJ.mm [ha.sup.-1] [h.sup.-1], resulting in an average of 1245 MJ.mm [ha.sup.-1] [h.sup.-1]. Average [I.sub.30] amounts to 7.2mm [h.sup.-1] and is therefore the smallest for whole of New Zealand. Mean storm erosivity in this region (33 MJ.mm [ha.sup.-1] [h.sup.-1]) accounts only 31-59% of the erosivity in the other regions. In contrast to eastern and inland South Island, which are the driest regions in New Zealand, annual precipitation in the southernmost part of western New Zealand is very high, but the rainfall occurs with much lower intensity than in western South Island. This results in relatively low erosivity values, which, together with the data from Eastern and Inland South Island, can be described by one regression. Therefore, these regions are combined to one. Rainfall is evenly distributed throughout the year; nevertheless, peak rainfall erosivity occurs in summer (Table 2).

Regions 1 and 3, both situated on the west coast of New Zealand, show similar relationship between rainfall and erosivity. Therefore, these two regions were described by one regression (Table 3).

Temporal rainfall erosivity analysis

The spatial distribution of annual rainfall erosivity in New Zealand as well as during spring, summer, autumn and winter are shown in Figs 2 and 5. The R contours range from <550 MJ.mm [ha.sup.-1] [h.sup.-1] in parts of Central Otago to >16000 MJ.mm [ha.sup.-1] [h.sup.-1] in the Southern Alps (Fig. 2); therefore, across both islands, rainfall erosivity varies by a factor of 30. Worldwide, only some regions have such high R-factors, such as the Amazonas region in Brazil (da Silva 2004) or in tropical regions (van der Linden 1983; Yu 1998).

The spatial pattern of rainfall erosivity during the four seasons is more or less the same. The most erosive storms occur during summer, leading to highest R-factors, whereas lowest erosivities are observed in winter (Fig. 5).

The erosivity density describes the ratio between mean annual erosivity and mean annual precipitation. It expresses the erosivity per rainfall unit in MJ [ha.sup.-1] [h.sup.-1] (Kinnell 2010). Areas of high erosivity density indicate that the precipitation is characterised by highly erosive events (storms). Figure 6 displays the spatial distribution of erosivity density over New Zealand. The most erosive storms occur in northern and western South Island as well as in northern New Zealand, indicated by the highest erosivity density of 2.1 (Table 2). For eastern and inland South Island as well as in southern New Zealand, the lowest erosivity density is obtained with a value of 0.7 which is only ~35% of the value of northern New Zealand and northern and western South Island. This high variability throughout both islands indicates that rainfall erosivity is not solely dependent on the amount of precipitation. Regional rainfall patterns with differences in rainfall intensity occur. The same three regions as found for rainfall erosivity can be distinguished (Table 3): (I) northern North Island and the Southern Alps, with erosivity density values >1.6 MJ [ha.sup.-1] [h.sup.-1]; (2) the remaining North Island, with values mainly 0.8-1.6 MJ [ha.sup.-1] [h.sup.-1], and (3) the remaining South Island, with values 0.4-1.2 MJ [ha.sup.-1] [h.sup.-1]. Based on these results, region-specific regression functions may be developed, which cannot be extrapolated to other areas with different climatic characteristics. Panagos et al. (2015) showed that erosivity density might also contribute to the identification of risk areas, when combining erosivity density and annual precipitation.

Figure 7 shows the temporal distribution of erosivity density throughout the year for the four regions. In all regions, a similar pattern but with different magnitude can be seen. Storms of a given amount are more erosive from December to April than during the rest of the year. They are ~2.1 times more erosive than winter storms (August-October, Fig. 5), which have the lowest erosivities.

According to Table 4, a large portion of New Zealand (89%) shows low to medium annual erosivity. Nevertheless, in 4% of the area, high to very high rainfall erosivity values (>7300 MJ.mm [ha.sup.-1] [h.sup.-1]) occur. This clearly demonstrates the importance of rainfall in erosion processes in New Zealand.

The rainfall erosivity map for New Zealand, in combination with recently published studies from Africa (Vrieling et al. 2014), Europe (Panagos et al. 2015), Australia (Lu and Yu 2002), Brazil (Oliveira et al. 2013) and China (Zhang et al. 2010), can be used for the development of a rainfall erosivity map of the world. All the above-mentioned studies used very similar methodologies for R-factor estimation.

Conclusions

The rainfall erosivity in New Zealand shows large spatial variability. Annual R-values vary across both islands by a factor of 30, from <550 MJ.mm [ha.sup.-1] [h.sup.-1] in parts of the south-western South Island to >16000 MJ mm [ha.sup.-1] [h.sup.-1] in the Southern Alps. This high variability of data is mainly related to climatic and topographic differences throughout both islands. Nevertheless, the data show a high correlation to the precipitation. A major part of the country (68%) revealed low rainfall erosivity, although >4% of New Zealand is affected by a high to very high erosivity. Storms with highest erosivities occur mostly during summer months. On average, these storms are 2.1 times more erosive than those that occur in winter.

The rainfall erosivity maps are useful to illustrate how rainfall influences soil erosion and represent an important source of information to predict erosion in New Zealand. Based on these maps combined with other information, strategies can be defined to minimise erosion in hotspot areas and to reduce offsite impacts of these processes.

http://dx.doi.org/10.1071/SR14363

Acknowledgement

The authors thank the New Zealand Institute of Water and Atmospheric Research (NIWA) for providing the data and the map of long-term annual precipitation.

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Andreas Klik (A,C,D), Kathrin Haas (A), Anna Dvorackova (B), and Ian C. Fuller (C)

(A) Department of Water, Atmosphere and Environment, University of Natural Resources and Life Sciences Vienna, Muthgasse 18, A-1190 Vienna, Austria.

(B) Czech University of Life Sciences Prague, Faculty of Environmental Sciences, Kamycka 129, 165 21 Praha 6, Czech Republic.

(C) Institute of Agriculture and Environment, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand.

(D) Corresponding author. Email: andreas.klik@boku.ac.at

Table 1. Location, data periods of investigated rain gauges,
number of erosive storms (>12.7 mm) per year (NES), annual
precipitation (P, mm [year.sup.-1]), average R-factors
(MJ.mm [ha.sup.-1] [h.sup.-1] [year.sup.-1), seasonal
distribution of P and R (%), mean R-factor of a single storm
event (Rss), average maximum 30-min rainfall intensity
([I.sub.30], mm [h.sup.-1]) and erosivity density (ED, MJ
[ha.sup.-1] [h.sup.-1]) for investigated rain gauges

S, Spring, Sept.-Nov.; Su, summer, Dec.-Feb.; A, autumn,
Mar.-May; W, winter, June-Aug.

Station                        Long.       Lat.      Elev.
No.        Name                                     (m asl)

North Island

1          Auckland           -37.2064   174.8638       88
2          Gisborne           -38.2857   177.5294      488
3          Hamilton           -37.7788   175.3127       40
4          Kaitaia            -35.1350   173.2620       85
5          Martinborough      -41.2523   175.3899       20
6          Matamata           -37.8768   175.7350      106
7          Mount Ruapehu      -39.1977   175.5449     1097
8          Napier             -39.6100   176.9120        5
9          New Plymouth       -39.3373   174.3049      300
10         Palmerston North   ^10.3820   17.,6092       21
11         Tarapounamu        -38.6184   176.8739      701
12         Taupo              -38.9950   175.8120      375
13         Wanganui           -39.9388   175.0450       15
14         Wellington         -41.1354   175.0528       56
15         Whangarei          -36.2730   174.7960       27
Mean
s.d.

South Island

16         Alexandra          -45.1243   170.1005      450
17         Arapito            ^41.2706   172.1557       18
18         Arthur's Pass      -42.9432   171.5628      738
19         Christchurch       -43.6262   172.4704       18
20         Dunedin            -45.9013   170.5147        4
21         Eglinton           ^44.9978   168.0180      365
22         Franz Josef        -43.3655   170.1343       80
23         Greymouth          -42.4602   171.1916        5
24         Hamner Forest      -42.5343   172.8510      362
25         Invercargill       -46.5870   168.3760        5
26         Lake Moreaki       -43.7188   169.2670       10
27         Lake Tekapo        -44.0017   170.4432      762
28         Manapouri          -45.5250   167.2750      178
29         Mount Cook         -43.7360   170.0960      765
30         Nelson             -41.3173   173.0948       18
31         Takahe Valley      -45.2960   167.6830      895
32         Takaka             -40.6364   172.8057       20
33         Timaru             -43.7935   171.7951      160
34         Waitutu            -46.1918   167.0661       30
35         Westport           -12.1170   171.8600      198
Mean
s.d.

Station      Data      No. of   NES    Ann.
No.         period     years            p

North Island

1          1997-2012      16    28.1   1188
2          2000-2012      12    40.3   2044
3          1997-2012      16    27.0   1096
4          1999-2012      14    32.6   1388
5          2001-2012      11    18.3   760
6          1999-2012      14    24.4   1037
7          2001-2012      12    55.2   2714
8          1997-2011      15    14.8   718
9          2003-2012      10    41.4   1869
10         2002-2012      11    23.8   982
11         2007-2012       6    36.5   1769
12         1997-2012      16    31.8   1462
13         1997-2012      16    22.5   927
14         1999-2012      14    29.7   1197
15         2000-2012      13    28.3   1209
Mean                    13.1    33.5   1357
s.d.                     2.7    8.0    522

South Island

16         2001-2012      12    oo oo  429
17         2006-2012       7    48.7   2160
18         2006-2012       7    59.0   4301
19         2000-2012      13    15.0   609
20         1998-2012      15    11.8   667
21         2010-2012       3    48.3   2169
22         2004-2011       9    58.8   4186
23         2003-2012      10    55.7   2284
24         1997-2011      15    21.4   962
25         1997-2012      16    25.4   1113
26         2005-2012       8    69.0   3939
27         2004-2012       9    11.6   523
28         2003-2012      10    63.5   3407
29         2001-2012      12    56.5   3846
30         2002-2012      11    21.8   922
31         2010-2012       3    56.0   2594
32         2004-2012       9    37.4   1985
33         1997-2012      16    16.7   711
34         2010-2012       3    34.3   2276
35         1999-2012      14    39.6   1717
Mean                    10.1    38.5   2040
s.d.                     4.0    19.0   1282

Station               Seasonal distribu  R-
No.                                    factor
            Sp     Su     A      W

North Island

1           25     20     23     31     2536
2           26     19     23     32     3505
3           23     23     23     31     1393
4           21     22     27     31     2816
5           23     23     22     32      477
6           23     25     24     27     2076
7           29     23     19     28     3592
8           18     21     27     33      776
9           23     22     23     32     3548
10          28     24     18     30      869
11          21     22     24     33     3008
12          27     23     22     29     1779
13          26     25     21     28     1144
14          27     21     20     32     1273
15          19     21     27     33     2569
Mean       24.0   22.3   22.8   30.9    2091
s.d.       3.1    1.7    2.6    1.9     1038

South Island

16          24     38     22     16      250
17          28     24     23     25     3871
18          29     25     24     23     8458
19          24     22     25     29      383
20          23     31     24     22      327
21          21     26     31     22     2069
22          26     30     21     22    10895
23          26     25     23     26     4139
24          27     19     23     31      611
25          26     23     27     24      618
26          25     28     25     22    10015
27          24     24     26     26      230
28          26     24     25     25     3757
29          26     29     25     21     8344
30          24     25     23     28     1448
31          26     24     34     16     2320
32          25     24     22     29     4819
33          23     26     23     28      429
34          22     24     29     25     2059
35          29     22     21     28     1891
Mean       25.2   25.6   24.8   24.3    3347
s.d.       2.2    3.9    3.3    3.8     3365

Station               Seasonal distribu Rss    [I.sub.30]    ED
No.
            Sp     Su     A      W

North Island

1           43     25     17     15    91.2         12.4    2.14
2           23     23     25     29    87.0         11.3    1.78
3           17     34     26     23    51.6         11.3    1.27
4           17     25     33     25    86.4         13.7    2.03
5           18     33     23     26    26.6          7.6    0.63
6           14     25     34     27    85.0         12.7    2.00
7           25     30     21     24    65.1          9.2    1.37
8           15     25     32     28    52.9          9.6    1.08
9           21     26     31     22    81.8         11.5    1.81
10          31     31     16     22    36.5          9.4    0.89
11          13     36     30     22    82.4         11.1    1.70
12          22     31     25     22    49.8          9.5    1.32
13          21     40     19     19    40.8         10.2    0.99
14          24     29     21     26    42.8          9.2    1.21
15          19     23     39     20    82.4         12.5    2.12
Mean       21.4   29.1   26.1   23.4   64.2         10.8    1.5
s.d.       7.3    5.0    6.6    3.6    21.3          1.6    0.5

South Island

16          10     72     15      3    28.3          7.7    0.58
17          21     27     29     24    79.5         11.3    1.88
18          29     31     23     16    138.3         9.7    1.97
19          18     35     27     20    25.5          6.9    0.63
20          17     47     22     14    26.9          6.9    0.49
21          12     34     35     19    40.6          7.3    0.95
22          22     37     24     17    164.8        13.2    2.60
23          17     26     31     25    73.5         12.1    1.81
24          23     27     25     26    28.6          6.6    0.64
25          23     29     30     18    23.6          7.1    0.56
26          21     34     28     17    133.0        13.8    2.54
27          16     29     25     30    18.8          5.9    0.44
28          24     29     25     22    59.3          7.6    1.10
29          19     42     28     12    94.9          9.9    2.17
30          14     31     30     25    57.6         10.6    1.57
31          18     33     34     16    36.9          7.1    0.89
32          19     30     24     27    103.4        12.6    2.43
33          17     40     22     20    26.2          6.9    0.60
34          19     30     31     20    37.8          7.7    0.90
35          24     31     22     23    47.4          8.6    1.10
Mean       19.2   34.7   26.4   19.6   62.2          9.0    1.3
s.d.       4.4    10.1   4.6    6.0    42.4          2.4    0.7

Table 2. Mean and standard deviation of number of erosive
storms (>12.7 mm) per year (NES), annual precipitation (P,
mm [year.sup.-1]), average R-factors (MJ.mm [ha.sup.-1]
[h.sup.-1] [year.sup.-1]), distribution of P and R, mean R-
factor of a single storm event (Rss), average maximum 30-min
rainfall intensity ([I.sub.30], mm [h.sup.-1]) and
erosivity density (ED, MJ [ha.sup.-1] [h.sup.-1]) for
different regions in New Zealand

Spring, Sept.-Nov.; summer, Dec.-Feb.; autumn,
Mar.-May; winter, June-Aug.

Region                      NES         Annual
                                           P

Northern New Zealand    28.3 (2.9)    1206 (124)
Rest of North Island    31.0 (11.2)   1413 (596)
  (central, eastern,
  south-west)
Western and northern    50.9 (13.9)   2953 (1186)
  South Island
Eastern, inland and     29.4 (17.9)   1431 (936)
  southern
  South Island
North Island                --        1361 (415)
  (339 stations)
South Island                --        1293 (1020)
  (258 stations)

Region                         Seasonal distribution            R-
                                                              factor
                        Spring   Summer   Autumn   Winter

Northern New Zealand    22 (2)   22 (2)   25 (2)   31 (2)   2499 (267)
Rest of North Island    25 (3)   22 (2)   22 (2)   31 (2)   1942 (1166)
  (central, eastern,
  south-west)
Western and northern    26 (2)   26 (2)   23 (1)   25 (3)   6498 (3157)
  South Island
Eastern, inland and     25 (2)   25 (5)   26 (4)   24 (4)   1245 (1091)
  southern
  South Island
North Island            24 (2)   21 (3)   25 (2)   30 (3)   2000 (966)
  (339 stations)
South Island            25 (2)   26 (4)   24 (2)   25 (4)   1538 (2635)
  (258 stations)

Region                       Seasonal distribution of R

                        Spring    Summer    Autumn   Winter

Northern New Zealand    23 (11)   24 (1)    31 (8)   22 (5)
Rest of North Island    21 (5)    31 (5)    24 (5)   24(3)
  (central, eastern,
  south-west)
Western and northern    20 (4)    32 (5)    27 (3)   20 (5)
  South Island
Eastern, inland and     19 (4)    36 (12)   26 (5)   19 (6)
  southern
  South Island
North Island            23 (3)    26 (5)    31 (5)   20 (3)
  (339 stations)
South Island            18 (5)    36 (8)    27 (3)   19 (3)
  (258 stations)

Region                      Rss        [I.sub.30]      ED

Northern New Zealand     86.3 (3.2)    12.8 (0.5)   2.1 (0.1)
Rest of North Island    56.1 (19.4)    10.0 (1.2)   1.3 (0.4)
  (central, eastern,
  south-west)
Western and northern    105.6 (34.4)   11.6 (1.4)   2.1 (0.4)
  South Island
Eastern, inland and     33.3 (11.0)    7.2 (0.7)    0.7 (0.2)
  southern
  South Island
North Island                 --            --       1.5 (0.4)
  (339 stations)
South Island                 --            --       1.2 (0.5)
  (258 stations)

Table 3. Type of regression (P, power function; L, linear
function) and coefficients describing annual as well as
seasonal rainfall erosivity

                       Region 1+3

          Type     a        b     [r.sup.2]

Year       P     0.773    1.130    0.9636

Spring     L     1.894      0      0.8699
Summer     P     0.674    1.207    0.9641
Autumn     L     2.533    -28.4    0.9112
Winter     P     0.749    1.12     0.8208

                       Region 2

          Type     a        b      [r.sup.2]

Year       P     0.026    1.536      0.903

Spring     P      0.08    1.435     0.9038
Summer     P     0.078    1.537      0.900
Autumn     L     2.508    -284.4     0.875
Winter     P     0.021    1.633      0.941

                       Region 4

          Type     a         b      [r.sup.2]

Year       P     0.043     1.397     0.9767

Spring     P     0.028     1.518     0.9688
Summer     P     0.159     1.324     0.9358
Autumn     L     1.100    -79.700    0.9657
Winter     P     0.007     1.775     0.9586

Table 4. Percentage of rain gauges and New Zealand's land
area w ith different rainfall erosivity classes

After Carvalho (2008) modified to S.I. metric units
according to Foster et al. (1981)

Erosivity class                                               Area
                                                              (%)
(MJ.mm [ha.sup.-1] [h.sup.-1] [year.sup.-1])

0-2452                                          Low           68.2
2453-1905                                       Medium        20.6
4906-7357                                       Medium-high    7.0
7358-9810                                       High           2.7
>9810                                           Very high      1.5
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Author:Klik, Andreas; Haas, Kathrin; Dvorackova, Anna; Fuller, Ian C.
Publication:Soil Research
Article Type:Report
Geographic Code:8NEWZ
Date:Oct 1, 2015
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