A spatial approach to rainfall in Sertão do Pajeú – Pernambuco

Changes in precipitation have implications for the hydrological cycle and water resources. Climate change is expected to alter average temperature and precipitation values, increasing the variability of these events, which could cause more intense and frequent floods and droughts. The objective of this study was to characterize the rainfall in the microregion of Pajeú, in Pernambuco, as well as to provide subsidies for public policies aimed at water scarcity. For this, rainfall data were used at stations belonging to the micro-region and its surroundings, for the period from January 1980 to December 2019. In addition, to mitigate the influences caused by temporal heterogeneity, stations with large discontinuity of information. The Inverse Weighted Distance was used to perform the interpolation of data and preparation of maps with isolines of rainfall. The results show the places with the highest annual rainfall during the study period were Serra Talhada and Triunfo, and the lowest rainfall occurred in the vicinity of Ingazeira and Tabira.


Introduction
Changes in rainfall behavior affect the hydrological cycle and conse-quently water resources. In cases of extreme climate change, the impactis related to changes in water resources, the occurrence of more severeand frequent floods and droughts (Silva et al, 2017). Therefore, it is relevant to carry out research studies on rainfall, they are important to support actions aimed at water resources, in particular the study region, which belongs to the semiarid region.
In the literature there are several data interpolation techniques. In Inverse Weighted Distance (IDW), the area of influence of each sample decreases as the distance increases in relation to another sample (Mazzini & Schettini, 2009). In this case, the sample weights during the interpolation technique are inversely proportional to the distance from the mesh node. (Santos et al., 2014).
In this context, Soares et al., (2014) were successful in applying the interpolation by IDW in his studies on the spatial and temporal distribution of precipitation in the hydrographic region of Ilha Grande Bay in the state of Rio de Janeiro. In turn, Santos et al., (2014) used the IDW to study the spatial and temporal variability of rainfall for the microregion of Pau dos Ferros, Rio Grande do Norte. Thus, the Inverse of Distance represents a commonly used interpolation technique.
Furthermore, the application of geostatistics is also applied to the data with interpolation techniques for construction of the estimated surface, in order to identify the best fit, as in the works of Carvalho and Assad (2005) In this sense, the research aims to characterize the rainfall in the microregion of Pajeú, in Pernambuco, as well as to provide subsidies for public policies aimed at water scarcity. In the next section, the method used for interpolation of rainfall is presented, considering data from the last 40 years.

Methodology
This is a quantitative, descriptive study, whose main purpose is to describe the characteristics of a given population or phenomenon (Sampieri et. al., 2013).

Study Area
The state of Pernambuco is subdivided into five mesoregions and nineteen microregions (IBGE, 2017). The study area is part of the Sertão mesoregion and corresponds to the group of municipalities in the Pajeú Pernambucano microregion with an approximate area of 8,770 km² and corresponds to 8.95% of the Pernambucano territory (IBGE, 2020).

Data
The data come from the National Agency for Water and Basic Sanitation (ANA), using information from rainfall stations, including stations belonging to the municipalities of the microregion of Pajeú and stations hosted in surrounding municipalities. Stations with great discontinuity of information were removed to mitigate the influences caused by temporal heterogeneity.
According to the World Meteorological Organization (WMO), for climatological studies, it is recommended that the historical series consider at least thirty years, in this way, average precipitation data from January 1980 to December 2019 were used. Finally, the treatment and data analysis using R and QGIS software.

Interpolation Technique
According to Yamamoto and Landim(2013), the process of reproduction of the characteristics of the spatial phenomenon based on sampling points is called interpolation or estimation. The interpolation of an unsampled point is performed by fitting local (points closest to the unsampled point) or global (all sampling points) mathematical functions.
For data interpolation, Inverse Distance Weighting -IDW was used, method proposed by Shepard (1968) as global interpolation method. This technique is frequently used and consists of assigning the greatest weight to the nearest point, decreasing the weight as the distance increases as a function of a power coefficient (α). Its mathematical formulation can be described as follows: Society andDevelopment, v. 10, n. 11, e241101119614, 2021 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v10i11.19614 where Z  corresponds to estimated values, x represents the number of samples, xi the known values, dij the distance between the points i and j, (Camargo, 1998).

Results and Discussion
Initially, the collection and processing of data was carried out, identifying the stations with information with greater constancy of information. Thus, for the analysis, data from twenty pluviometric stations were used, among which thirteen belonging to the microregion of Pajeú and seven were located in surrounding municipalities, including the municipality of the State of Paraíba, Tables 1 and 2, respectively.  With the interpolated map, it is possible to identify the spatial distribution of rainfall in the Pajeú microregion, as well as to identify the territories with the highest average rainfall, such as the Serra Talhada and Triunfo municipalities, and the places with lower annual rainfall such as in the boundaries between the municipalities of Tabira and Ingazeira (Figure 2). Source: Authors (2020).

Final Considerations
The interpolation of pluviometric precipitation projects a possibility of using this technique as a tool for the management of water resources in the territory. Based on the use of spatial geotechnology tools with IDW interpolation, the technique proved to be adequate to extract relevant information regarding studies related to the characterization of rainfall in the microregion of Pajeú, in Pernambuco.
Finally, the pluviometric spatial description of the territory provides subsidies for public policies aimed at water scarcity. Thus, in addition to the methodology used in this study, for future work, Geostatistics, as well as other interpolation methods, can also be considered to assist in planning and monitoring the operational development of rainfall networks.