Long-Term Variation of Sunshine Duration and Their Inter-Action with Meteorogical Parameters over Chad, Central Africa

This study aims to evaluate the long-term variations of sunshine duration and to estimate its interaction with meteorogical parameters from 1950 to 2010 in Chad, Central Africa. The results show that Chad is the one of world’s sunniest countries. Each year, Chad receives more than 3030.91 ± 176.33 hours of sunning corresponding to 8.9 hours daily. Likewise, a strong north-south gradient is noted over Chad. For instance, the daily insolation is 10 ± 0.41 hours in the north, 8.85 ± 1.1 hours in the center and 7.75 ± 1.8 hours in the south. Furthermore, there is a marked seasonality of sunshine duration with maximums in dry season and minimums during the rainy season. The lowest values of sunshine duration are found in August. On the contrary, maximums are recorded from November to February with values greater than 9.5 hours per day. Moreover, the annual anomalies study allowed determining three great periods in terms of sunshine variability in Chad. Firstly, the period between 1950 to 1970 (named humid period) is characterized by the lowest values of sunshine duration in Chad. Secondly, from 1970 and 1990, the region suffered an unprecedented drought which resulted in an increase of sunstroke duration in Chad. And finally, the period from 1990 to 2010 called the return period is characterized by a rapid year-to-year fluctuations of insolation duration. Unlike to surface temperature, we have also shown that there is a direct relation between the duration of insolation and the meteorological parameters such as precipitations and relative humidity.


Introduction
Sunshine is a key factor in most climate processes e.g. Earth's radiative balance and hydrological cycle (Ranzi et Rosso, 1995) [1]. The sunshine hour is the most important influencing parameter of evapotranspiration process (Pandey et al. 2014(Pandey et al. , 2016 [2] [3]. Thus, Sunshine duration variations can disrupt the hydrological cycle by reducing evaporation, resulting in reduced precipitation and agricultural yields in some areas. Overall, Sunshine duration has been used for decades in many different applications. It is applied in sectors such as tourism, public health, agriculture, vegetation and solar energy ( (Dobesch, 1992) [10]. In Western Europe, the annual sunshine duration showed an overall decrease since the 1950s until the early 1980s, followed by a recovery during the next two decades (Sanchez-Lorenzo et al., 2008) [11].
However, few studies have been published on sunshine duration in Africa despite its good solar potential (Babikir et al., 2018) [7]. Indeed, the sunshine duration has a fairly direct relationship with the solar radiation received at the surface. Consequently, the knowledge of the Sunshine variability represents a determining factor for design, dimensioning, performance assessment, and energetic management of renewable energy conversion systems for countries devoid of solar radiation measurement. The focus of this work is on a Chad, country located between Sahel and Central Africa. Its climate is characterized by warm and dry continental type. From north to south of Chad, we have a succession of Saharan climate in the north, Sahelian in center and Sudanese in the South (Bedoum et al., 2013, Maharana et al., 2018 [12] [13]. As most Sahelian countries, Chad endured an intense drought from the early 1970s to the late 1980s. This drought has been manifested by a rainfall deficit that has had a negative impact on the region agriculture and economy (Lebel et al., 1996 [23]. One of the major questions is to know if the recently flucatuations of Africa climate (great drought of the 70s and recently global climate change) have had an impact on sunshine duration. The aim of this work is to study the long-term variability of sunshine duration and its interaction with climatic parameters in Chad, Central Africa. The paper is organized as follows. "Data and Methodology" presents the study area (the map of Chad with her 7 ground stations), the datasets used in this work and the methodology. The results are presented in "Results and discussions".

Datasets Collection
In this study, in situ data of sunshine duration, precipitation, humidity, Tem-  The usefulness of this system is that all stations include a monthly time series of good quality over the considered period.

Methodology
In this study we have subdivided Chad into three zones according to the climate of the ground station. Therefore, the northern zone is composed of one station, the central zone of 3 stations and the zone south zone of 3 stations. For each zone, we average the data of different stations to obtain a data by climate zone.
In addition, the data representing the whole country are calculated with the av-

Annual and Seasonal Characterization of Sunshine Duration
We begin by shown in the Figure

Effect of Seasonal Variability of Meteorological Parameter on Insolation Duration
In order to assess the annual impact of meteorological parameters on sunshine duration, we begin by analyzing the Standardized precipitation index (SPI) in Chad from 1950 to 2010. Figure 5 presents annual anomalies of insolation ( Figure 5(b)) and precipitation (SPI index in Figure 5    We note that, the relative humidity follows the same trend as the sunshine duration. Indeed, insolation is a parameter that depends on relative humidity and therefore the cloud cover. We recognize the signature of the three major climatic periods of Chad. In fact, the positive anomalies of relative humidity ( Figure   6    drought period and to −0.28 for the return period. Between the wet and dry period, there is an increase for average and minimum temperatures; and a maximum temperature increase. These changes are probably due to climate variability at the regional scale. However, between the dry period and return, the temperatures increase considerably. These increases are due to global scale climate change and therefore does not have a strong impact on the duration of sunshine.

Conclusions
The objective of this study was to evaluate the long-term variability of insolation duration and its interaction with meteorological parameters such as surface This shows the regional aspect of these three parameters which are: insolation duration, precipitations and relative humidity. However, the relationship between temperatures and sunstroke duration is not direct, especially during the period of return of the rains between 1990 and 2010. During this period, temperatures increase rapidly with the emergence of regular heat waves, mainly due to the global climatic changes.