Characteristics and tendencies of the Gumanti Valley rainfall pattern towards erosivity as a mitigation effort in facing the threat of climate hazards

. Rainfall is one of the most important climatic parameters in the agricultural sector. Each raindrop contains kinetic energy that can destroy soil particles and carry them away to lower places. Nutrients located in the top layer of soil are susceptible to erosion, which decreases soil productivity. Rainfall patterns are formed due to global, regional, and local factors that influence them. Changes in rainfall patterns will affect erosivity. The rainfall data used in this study came from the cooperation rain post of the West Sumatra Climatology Station located in Lembah Gumanti. Linear regression was used to assess the rainfall trend and Bols erosivity to calculate the effect of rainfall on erosion. The trend of rainfall in the Lembah Gumanti from 2014–2022 showed a value of +0.195, and the largest erosivity value occurred in 2020 as a result of the La Nina climate phenomenon. This research shows that an increase in rainfall will be followed by an increase in erosivity. High erosivity is feared to threaten the availability of soil nutrients, so mitigation efforts need to be made in anticipation of the threat of climate hazards for agricultural areas located in the highlands, such as the Lembah Gumanti region.


Introduction
Climate variability is an anomaly or deviation from the average temporal (month, season, and year) and spatial scales.In assessing climate variability, we can use a region's longest climate history data.One of the climate parameters that is closely related to the agricultural sector is rainfall.Water functions as a carrier for soil nutrients to be absorbed by plants.In addition to these functions, water can also have a negative impact, namely as a factor causing erosion.The higher the amount and intensity of rain, the greater the surface runoff, so the eroded soil also increases.Erosion will cause the top layer of soil to erode, and the particles will be carried away to a lower place [1,2].
The nutrients are located in the top layer of the soil, a layer that is prone to erosion.Continuous erosion will cause nutrients to be washed away, resulting in decreased agriculture [3].In addition, changes in land use due to human activities and inappropriate land management impact the amount of variation and spatial distribution of erosion.This will decrease land productivity, threaten food security, and increase socio-economic [4,5].Increased erosion by rain has significantly impacted tropical countries in West Africa and Asia, including Indonesia [6].
The climatic factor that most determines erosion is rainfall expressed in the "rainfall erosivity index".The erosivity shows the ability or capacity of rain to cause soil erosion [7].The erosivity factor of rain is the result of multiplying the kinetic energy (E) of one rain incident with a maximum rain intensity of 30 minutes (I30) [8,9].
The amount of soil released per one drop of rain that splashes is directly proportional to the drop's size, speed, and intensity.The factors that affect the erosivity are the amount, intensity, velocity, grain size, and distribution of the size of the falling raindrops [7,9].Rain that falls to the ground has energy that can be divided into potential energy and kinetic energy.Kinetic energy occurs when rain falls to the ground at a certain speed and destroys soil aggregates.The exact amount of rainfall can have a different effect on erosion depending on the intensity and condition of the soil surface [7].On agricultural land located in the highlands with hilly natural formations, such as the Gumanti Valley, erosion is affected by rain and the slope of the land.The greater the slope angle of the land, the greater the erosion potential.The slope of the slope and the length of the slope have an impact on the surface runoff rate, which carries the topsoil and nutrients from one lower place to another [10].
The pattern of rainfall every year in an area is not the same.It is influenced by global, regional, and local phenomena.ENSO (El Nino and La Nina) is a global phenomenon affecting rainfall in Indonesia [11].Madden Julian Oscillation, monsoon, convergence, topography, and regional morphology influence rainfall patterns at regional and local scales.El Nino events are usually followed by a decrease in rainfall and an increase in air temperature, while La Nina events stimulate an increase in above-average rainfall.Both climatic anomalies are unfavorable for agricultural production because a drastic decrease in rainfall due to El Nino can cause crop failure due to drought.In contrast, increased rainfall due to La Nina can cause flooding and increase plant-disturbing organisms' disruption [12].
Changes in rainfall patterns due to these climatic phenomena will contribute to the size of the rainfall erosivity value [13][14][15].Therefore, this study aims to determine the effect of rainfall pattern characteristics on erosivity in dryland agricultural areas in Gumanti Valley District, Solok Regency.This research is expected to be used as a guide for mitigating dryland agriculture in hilly areas against climate threats.

Data
The research location is located in Solok Regency, Lembah Gumanti District, at coordinates 01.5718 -01.1323''South Latitude, and 100.4448 -100.5545''East Longitude.This area is a plateau with a stretch of land in the form of hills.The altitude is 1.382 -1.458 m from sea level and the temperature is about 17°C -19°C.The rainfall data was used from the Gumanti Valley rain station for the period 2014 -2022 (BMKG).The rainfall in the region is high with annual rainfall about 2.000 -2.500 mm/year.The climate is very favorable for horticultural agriculture.In general, dry land farming in this area involves planting onion, chili, tomatoes, cabbage, etc in monoculture, multi-cropping, and rotation.(Figure 1).

Methods
The method used in this study is linear regression analysis to examine the trend of rainfall characteristics and Bols (1978) for calculating erosivity.Filling in incomplete rain data is done by the average method.Linear regression analysis is a useful statistical technique for examining and modeling the relationships between variables.A positive value means an upward trend and a negative value means that there is a downward trend [16].The trend of increasing rainfall patterns means that there is a tendency for rainfall to increase from time to time and vice versa.
Where y is the dependent variable, x is the independent variable, and a and b are constants representing the slope of the regression.The erosivity value of rain is calculated using the Bols (1978) method with the equation: R=6,119 (CH) 1,21 (HH) (-0,47) Where R is the erosivity, CH is the monthly rainfall (cm), HH is the number of rainy days in one month, and CHmax is the maximum daily rainfall for 24 hours in that month (cm).while the dry season period occurs in January-February and July-September, with an annual average rainfall of 2,000-2,500 mm/year.(Figure 2) The trend is the tendency of the parameter value to change up or down in a certain period.Rainfall trend analysis for the period 2014-2022 shows a positive trend (0.195 mm/month).This value shows a trend of increasing rainfall by 0.195 mm every month in the Gumanti Valley.(Figure 3).

Fraction of rainfall and number of rainy days
The rainfall fraction is the ratio of certain amounts of rainfall to rainy days.Rain is categorized as heavy if its intensity reaches 50 mm/day.Based on Figure 4, the trend of the 50 mm rain fraction is greater than the 20 mm rain fraction, namely +0.07 and +0.06.Thus, the tendency to rain 50 mm/day is greater in the future.(Figure 4).Rainy days are the number of days when it rains with an amount of rainfall greater than or equal to 1 mm. Figure 4 shows the trend of rainy days based on a certain amount of rainfall.Rainy days with rainfall > 1mm and > 20mm have a tendency of -0.05 and -0.1, meaning that there is a decrease in rain events with these values.Meanwhile, the trend for the number of rainy days with > 50 mm of rainfall was +0.05, meaning that the number of rainy days with > 50 mm of rainfall per day increased in that period.Thus, the positive trend of 50 mm rain fraction and rainy days (> 50 mm) is feared to cause greater erosion potential, which will decrease agricultural land productivity in hilly areas.(Figure 5)

Erosivity
Based on Figure 6, the average erosivity value per year is 1.711,7, with the highest erosivity occurring in 2020 at 2.245,7 and the lowest erosivity occurring in 2021 at 1493,2.Increased rainfall due to the global La Nina factor is one of the triggering factors for high erosivity in 2020.As in Figure 7, strong El Nino (2015) came in the wet season in this area, but there was no significant effect on rainfall.The erosivity in this year had not decreased.But in 2020 moderate La Nina came in the wet season, then had a significant effect on rainfall.This analysis shows that La Nina has a strong influence on erosion, while La Nina occurs in this period erosivity rises significantly.
The erosivity trend from 2014-2022 has a regression equation y = 5,99x + 10373.The positive constant a represents an increasing erosion trend of 5,99 per year.An increase in rainfall (amount, rainy days, max rain) will be followed by an increase in rainfall erosivity.Sugesti, 2017 in her research stated that the erosivity calculation of erosivity in Padang City using Bols (1978) has the highest correlation (0.885) compared to Lenvain (1989) and Utomo (1989) [17].(Figure 8)  The average monthly erosivity for the 2014-2022 period was highest in November and lowest in July at 285,5 and 67,5.Based on Figure 9, the erosivity value appears to follow the rainfall pattern in the Gumanti Valley.When the rainfall is high, it will be followed by an increase in the erosivity value, and vice versa.

Number of rainy days Gumanti Valley
The relationship between rainfall, rainy days, and daily maximum rainfall to erosivity values shows very good correlation values of r = 0.97, r = 0.88, and r = 0.89.Based on this correlation value, rainfall has a greater influence than rainy days and daily maximum rainfall erosivity.

Mitigation efforts
Gumanti Valley is a highland area that is one of the horticulture centers in West Sumatra.This area is located on slopes that vary from flat to very steep, with relatively high rainfall of 2,000-2,500 mm/year, making it vulnerable to erosion due to rain.The tendency of increasing rainfall due to climate variations will have an impact on the erosion of agricultural land, so mitigation efforts are needed so that sustainable agriculture can be realized.To maintain agricultural productivity, farmers are expected to carry out soil conservation efforts.Among the soil conservation efforts recommended by Balitbangtan [18] for farming with raised beds in the direction of the slope are the following:

A. Mulcing
The use of plastic mulch is widely practiced by vegetable farmers because it is easier and more efficient.This method can compensate for the cost of farming because maintenance and weeding costs are lower.The use of plastic mulch can overcome the problem of using labor for weeding because weeds cannot grow under the humidity of the plastic.The soil is maintained, and there is no erosion or destruction of the soil surface so that erosion does not occur.2002) found that the beds were made in the direction of the slope and every 4.5-5.0m were cut with terrace mounds to reduce the amount of eroded soil.The amount of erosion when planting beans and cabbage on Hapludands Pacet, Cianjur, with a slope of 9-22% decreased by 28-38% compared to the amount of erosion on the beds in the direction of the slope.The results of this study indicate that the amount of erosion on beds along a slope with a length of 10 m is 2.5 times greater than the amount of erosion on beds with a length of 4.5 m in the direction of the slope cut by the mound.terrace at the bottom end of the bed.Besides that, In the field of terracing bunds, it is recommended to plant plants that are useful and have good selling value as a substitute for the loss of land used for making bund terraces, even though these two types of plants do no effect on reducing runoff and erosion rates [19][20][21].
The results of the study, also carried out by Erfandi et al. (2002) on Andic Eutrudepts soil in Campaka, Cianjur Regency, showed that raised beds with a length of 5 m in the direction of the slope were cut by mound terraces, and the beds made in the same direction as the contours could significantly reduce the amount of runoff and erosion.Compared to the seam along the slope, the amount of erosion in the area is reduced by 50-70% in the 5 m layer along the slope and 90-95% in the layer along the contour [21].Erfandi et al. (2002) also examined yields carried out on land with 5 m beds in the direction of the slope, namely the yield of red beans, beans, and cabbage from planting fields with 5 m beds in the direction of the slope and beds along the contour, which was almost the same as the yield of vegetables from the beds in the opposite direction.The same.Slope.However, in the long term, the yield of vegetable crops in the 5 m-long bed along the slope is cut into mound terraces.Beds in the direction of the contour are expected to maintain soil productivity because the thickness of the topsoil, which is useful for plant growth, is maintained because little soil is eroded [21,22].

Conclusion
Global, regional, and local climate phenomena will affect rainfall [23].The increasing trend of rainfall both in terms of amount and fraction can be used as an early indicator to implement soil conservation measures in order to minimize erosion caused by rainwater.The moderate La Nina phenomenon that occurred in 2020 significantly affected the increase in rainfall erosivity compared to other years, reaching 2.245,7.The correlation of rainfall with erosivity reached a higher value than rainy days and daily maximum rainfall [24,25].In general, in November the erosivity in the Gumanti Valley is high compared to other months because November is the month of peak rainfall.Recently, global climatic factors have not occurred periodically due to climate change, so mitigation efforts are needed to avoid the negative impact of rainfall on erosion.
Soil conservation efforts must be adjusted to the abilities of farmers.Farmers in the Gumanti Valley have used plastic mulch in their agricultural practices.It would be even better if farmers applied the concept of cutting beds every 5 meters in the direction of the slope with terraces or guluds so that erosion that occurs can be minimized.

Fig. 3 .
Fig. 3.The trend of rainfall in the Gumanti Valley.Based on the average rainfall of the Gumanti Valley Post for the 2014-2022 period, this area experiences two seasons, the rainy season and the dry season.The rainy season is the period of rainfall of 150 mm/month and the dry season is the period of rainfall of less than 150 mm/month.Thus, the Gumanti Valley enters the rainy season period in October-December and March-June,

Fig. 4 .
Fig. 4. Fractions and trends of 20mm and 50mm rainfall in the Gumanti Valley.

Fig. 10 .
Fig. 10.Installation of plastic mulch on shallot fields in the Gumanti Valley.B. Cutting the raised bed with a mound every 4.5 to 5 m Cutting the beds on agricultural land towards the slope aims to minimize runoff.Suganda et al. (1997), Haryati et al. (2001), and Erfandi et al. (2002) found that the beds were made in the direction of the slope and every 4.5-5.0