Spatial and temporal rainfall variability and erosivity: Case of the Issen watershed, SW-Morocco

. The Issen basin is a dry area affected by climate variability and desertification. It is located in high spot of Moroccan flora biodiversity that includes argan tree ecosystem as a part of the Western High Atlas (WHA). The objective of this work is to assess rainfall variability and erosivity as determining factors of soil erosion and vegetation cover degradation. The results reveal an arid to superior semi-arid climate where annual rainfall range from 200 mm in the centre of the Argana corridor, to 500 mm in the snow-covered northern foothills. The analysis of the 37 years of rainfall data indicate a coefficient of variation Cvis about67%. The erosivity index presents a high variability range from 34 to more than 81 MJ.mm/ha.h.yr in the wettest areas, indicating considerable soil erosion. In addition, the basin suffering from severe periods of drought due to the occurrence of the continuous decrease of the accumulated precipitation. Eventually, the rainfall variability and erosivity can affect ecosystem function and services (ecosystem degradation, water availability, etc).


Introduction
The Moroccan country characterized by three different climatic systems; the humid Mediterranean and humid Atlantic climates in the north and the arid Saharan climate in the south [1]. During the last decades, it was affected by a longest dry period which characterized by rainfall decrease and temperatures increase [2]. In fact, 53% of the total country was affected [3]. The rainfall decrease affects the vegetation cover growth, especially, in the Moroccan High Atlas mountains [4].
The ecosystems and agriculture activities in the semi-arid areas principally depend on the water availability [3]. In fact, the analysis of the rainfall data is crucial for assessing current environmental constraints and for optimal management of the limited water resources [5].In general, the climate change amplify existing risks and generate new ones for natural and human systems [6].
The Issen basin is the largest sub-basin in the right bank of the Souss river which covers an area of 1302 km² ( fig. 1). The Issen river is the most important tributary descending from the Western High Atlas (WHA), and it is a part of the Argana corridor and its annexed *Correspondingauthor:medaithaddou@gmail.com valleys considered as a suitable area to characterize rainfall instability in the High Atlas Mountains [7]. Geologically, the basin cover all Permo-Triassic red sandstone, detrital sandstone and conglomeratic silty sandstone Formations, which including at least 2500 m of fluvial, lacustrine and clastic deposits and coarse to fine floodplains [8]. Based on the national soil map database (EuDASM) [9], the red mountain forest soils and the brown and red lightly forested podzolic soils with very heterogeneous vegetation coverare developed on the Triassic Formations. The depression between the ancient massif and the western plateaus creates a natural passageway for the air currents which developing a particular and complex climatic conditions [10]. Topographically, the study area is a sloped land (the average slope is about 33%). In addition, the study area is a fragile environment influenced by the pedoclimatic hazards and the frequency of atmospheric disturbances, which affectingthe vegetation distribution (Fig.  2). Also, the study area is characterized by the presence of the argan steppes. Due to the climatic aggressiveness andthe high anthropogenic pressure, the High Atlas mountains and the Issen basin as a part of it suffering from water erosion risks, including soil degradation and flooding [11,12].
The vegetation cover in the Issen basin includes the argan tree (Arganiaspinosa) ecosystems, which is considered as a natural, national and universal heritage, associated with endemic or rare plants serving several applications; aromatic, medicinal, melliferous and pastoral [13]. The present study aims to analyse the contemporary spatial and temporal variability of rainfall and its associated risk responsible of the simple and combined climatic indices commonly used in Mediterranean countries as useful metric parameters to describe the climat may induces perenity of the remarkable endemic flora species and the water resources stored in the dams that ensuring the drinking water Mm³) and the agricultural irrigation for the Is mapped the spatial distribution of rainfall and the erosivity in order to determine the susceptible areas to water erosion which will be helpful to manage the erosion risks.

Data availability
Climatic data were obtained from the regional climate dataset compiled by the Massaand Tensift Hydraulic Basin Agencies South-West and High Atlas HCEFLCD (Tab. 1 and 2). the period of 37 to 50 years. In this study, we analyzed the data covering a satisfactory number of years without missing data. the Issen basin [14].
The present study aims to analyse the contemporary spatial and temporal variability of responsible of the soil degradation. Practically, we calculated and combined climatic indices commonly used in Mediterranean countries as a climate variability. In fact, the climate variability may induces perenity of the remarkable endemic flora species and the water resources stored in the dams that ensuring the drinking water supply for the Agadir metropolis (9.5 Mm³) and the agricultural irrigation for the Issen perimeter (13,000 ha) [15]. Moreover, we mapped the spatial distribution of rainfall and the erosivity in order to determine the susceptible areas to water erosion which will be helpful to manage the erosion risks.
Climatic data were obtained from the regional climate dataset compiled by the Souss ies (ABH) and the stations under control of the HCEFLCD (Tab. 1 and 2). The collected rainfall data covering period of 37 to 50 years. In this study, we analyzed the data covering a satisfactory The temperature is essential data because it is influences the spatial distribution of the vegetation. Table 2 shows the maxima (M), minima (m) and monthly means (Tm) of temperatures in °C of the three stations. Table 2.Thermal characteristics of the Abdelmomen, Argana and Immouzzerstations.

Data preprocessing
First, based on the monthly and annual averages of the collected rainfall data the missed values was deduced in order to complete datasets. Secondly, the monthly and annual averages were also used to calculate the climate indices (seasonal regime, monthly drought, deviation-to-mean index (Em), Standardized Precipitation Index (SPI) and rainfall erosivity (R-factor)). Finally, all the annual averages of precipitation and R-factor values were integrated and analyzed in a GIS tool. In fact, mapping rainfall distribution and erosion risk areas was achieved using interpolation krigging method.

Rainfall variabilityand trends
The rainfall deficit, the extent of the drought and its intensity were evaluated using thefollowingindices:

2.3.1Monthly drought
The monthly drought have been assessed based on the total rainfall (P) and the average monthly temperature (T). [16] reported that a month be dry if P ≤ 2T. In other hand, [17] indicated that if P < 3T or P < 4T the month is dry.

Deviation-to-mean index (Em)
Emis the most used index to estimate the rainfall deficit [18,19] (equation 1). It is positive for wet years and negative for dry years.

Em = Xi -Xm(1)
Where: Xi is the cumulative rainfall for year i, and Xm is the mean annual precipitation

Standardized Precipitation Index (SPI)
The Standardized Precipitation Index (SPI) was developed by Mckee et al. [20]. The SPI have been used to verify humid periods as well as dry periods according to equation 2.

SPI = (Xi -Xm) / Si(2)
Where: Xi is the cumulative rainfall for year i, Xm is the mean annual precipitation, and Si is the standard deviation of the time series annual rainfall. Effectively, several studies have been used the SPI and defined the different classes of drought severity [5,18,19,21]. The negative annual values indicate a dryperiod and positive values indicate a humid period (Tab. 3).

Rainfall Erosivity R-Factor
The rainfall erosivityis critical factor to assess the soil erosion risks [22].The rainfall erosivityis the product of the energy kinetics of rainy event and its maximum intensity in 30 minutes (IE30) [23].Wischmeier& Smith [24] provided a complete description of the rainfall erosivity. Arnoldus [25] proposed the Modified Fournier Index (IFM) which combines both the monthly average rainfall for all months of the year (Rm) and the annual average (Xm) (equation 3).
In this study, due to do not have sufficiently rainfall stations we used linear interpolation to generate the erosivity map. In general, the interpolation method guarantees the minimum variance topredict unknown values.
3 Results and discussion 3.1 Rainfall variability and trend

Annual and seasonal average
The isohyets of the Issen basin range between 100 and 700 mm (Fig. 3). The analysis of the obtained map reveals a high spatial irregularity of the total precipitation. In general, therainfall increasesfrom the center of the Argana corridor to its borders in relationship with altitude.In addition, the rainfall is lower and relatively homogeneous in the center of the basin. The rainfall ranges from 100 to 350 mm/year because they are well protected by the high plateaus of Tanana which brock up the oceanic effects. WHA massif (the snow-covered northern foot an average precipitation more than 400 mm/year.  . 4a), the arithmetic mean of rainfall is associated with a coefficient of variation Cv = 51% during great fluctuation and variability of rainfall. The most remarkable peaks are and 09/10 which represent the wettest years, year. The whole basinreceives an average spring. However, dry season is summer during it the basin receives of the annual precipitation (Fig. 4b).
The spatial and temporal precipitation variability which increasing soil erosion and then environment degradation  Table 4 resumes the seasonal regime in the Issen basin. The winter season is the wettest, which represents 50% of the annual total. The maximum winter rainfall is about 271 and 246 m observed in the Immouzzer and Aghbar stations, respectively. The summer season is the driest, which represents only 1 to 6% of the annual total. The seasonal regime was defined to classify the seasons in descending order of precipitation.The seasonal rainfall regime in the majority of the basin is of the WASS type (Winter-Automn-spring and summer). All the stations representing a summer deficit (m2) and a winter maximum (M1) are subject to the "Soussien" currents in the south-west. The only exceptions are the very northern stations, Timlilt, T.O.Maacho and Iloudjane, that are WSAS type (winter-spring-automn and summer), which show a second maximum (M2) in spring as opposed to autumn and are subject to the "Haouzien" currents of the north-east (Fig. 5). The transition between the two types of regimes occurs in the high basin of Issen at the upstream of the Tizguine station. The ombrothermal diagrams for three stations show that the whole basin is a drought season lasting 5 to 7 months and a thermal amplitud continental conditions (cold winter Embergerclimagram, the study stations are located in the arid bioclimatic zone with a temperate winter and in the semi-arid bioclimatic zone

Deviation-to-mean index (Em)
The results show very unequal spatial and temporal variation normal and wet years. The stations of Tizi and Dkhila(downstream part) present an average of drought years of more indicates a tendency to a dryness. The stations of Argana, Abdelmomen, Aguenza, Ain asmama and Immouzzer recorded annual averages of normal rainfall over 20 years. However, a limited number of the stations receive with a frequency of wet years over 35%. We droughty (tab.5). The long period of drought in the study area growth and development. The ombrothermal diagrams for three stations show that the whole basin is characterized by 5 to 7 months and a thermal amplitude characterizing semicontinental conditions (cold winter -very hot summer). According to the Embergerclimagram, the study stations are located in the arid bioclimatic zone with a arid bioclimatic zone with a cold to worm winter.
very unequal spatial and temporal variation of the drought frequency, normal and wet years. The stations of Tizi-Ou-Maacho, Ikakern, Tizguine(upstream part) present an average of drought years of more than 55%, which indicates a tendency to a dryness. The stations of Argana, Abdelmomen, Aguenza, Ain asmama and Immouzzer recorded annual averages of normal rainfall over 20 years. stations receivesannual totals above the habitual average with a frequency of wet years over 35%. We notice that more than 40% of the years are long period of drought in the study area may affect the vegetation , normal and humid years during 1980-2017    Figure 6 shows that the Issen Basin has suffered from dry episodes during 1981, 1986, 1992, 1998, 2002, 2006, 2012 and 2014-2017. The analysis of these results indicates a higher frequency of drought years in comparison to the humid years. In addition, the annual droughts occur once or twice every 10 years. In other hands, the drought that lasting 5 years may return after 360 years [27].

Conclusions and recommendations
The study investigated the indicators of drought and climatic aggressiveness in the Issen basin in order to characterize the spatial and temporal variability of precipitation and evaluate the rainfall erosivity. In fact, the record data collected from 13stations over the study area covering the period 1980-2017 were analyzed using a GIS tools. Thefindings revealed that almost all the studied stations recorded a rainfall below 500 mm with an arithmetic mean of 302 mm over the whole basin. The record data of 10 stations (WASStype) reflecting a high aridity and inter-annual and seasonal variability in the study area. In detail, the aridity extends from the beginning of April to the end of October and the dry season duration is about six to seven months per year. Furthermore, the dry periods have been more frequent and remarkable in recent years than humid periods.The analysis of precipitation variabilityusing standardized precipitation index (SPI) indicates moisture deficit.In fact, 70% of the study area is affected by moderate rainfall erosivity which is responsible of ecosystem degradation.
The occurrence of a succession of drought, climate events and the rainfall aggressiveness, especially, during the last decade threaten water security, the resilience of the argan trees ecosystem (southwest part of the basin), the quality of the soil and the agricultural sustainability in the study area. In fact, the soil erosion problem can be solved by constructing other dam in the upstream part of the basin which is the most part affected by water erosion. The action will also conserve drinking and irrigation water resources. In addition, it is recommended to regenerate the natural forests such as the argan forest, which is will be helpful to reduce the climate vulnerability. Moreover, extending and serving the researches continuity in the whole area of the Western High Atlas (WHA) can be helpful to figure out the precipitation trend.