## 2 Materials and Methods
### Methods
#### 2.2.3 Basin Morphological Characteristics  
There are several methods to determine the geomorphological indices of a basin under study, such as area, perimeter, slope, length of the main channel, and time of concentration. This is because urban basins of known regular shapes, such as square, rectangle, triangle, and circle, can be found. These types of basins can be analyzed with a relatively easy calculation method. However, it is necessary to work with irregular shapes, that is, with no known geometric shape, due to the variety that could be present in the municipality of Culiacan [32]. In this study, the hydrological analysis tools of QGIS 3.34.0 'Prizren' software were used to calculate the indices of the basic morphological characteristics of the basin under study. The basin perimeter was calculated using the \"watershed basin analysis\" algorithm, which identifies and connects the highest elevation points surrounding the area's surface drainage network, generating a polygon that is the basin divide or water parting. Subsequently, its perimeter in meters and its area in km\({}^{2}\) were obtained using the field calculator, with the results outlined in a polygon attribute table. The length of the mainstream was obtained by manually tracing polygonal lines along the longest streams in the basin, consulting their lengths in the attribute table of each line, and selecting the longest one. The slope of the mainstream was calculated by dividing the difference between the maximum and minimum elevations by the length of the line. The values of these variables were obtained using the \"profile tool\" algorithm. In turn, the time of concentration is equivalent to the time it takes for water to pass from the farthest point to the basin outlet [33]. To calculate it, the Kirpich formula was selected due to its easy application [34]. 2.2.4. Analysis and Processing of Rainfall Records  
It can be stated that the magnitude of rainfall is proportional to the magnitude of surface runoff. This is the reason why drainage studies rely on the study of precipitation to calculate design flows [35]. The measurement of precipitation is mainly carried out by means of rain pluviometers and pluviographs that record the amount of rainfall (mm). Due to the large territorial extensions of modern cities, it is desirable to have several stations in the same urban area. However, in Culiacan, there is only pluviometric information available from one of the climatological stations of the National Network of CONAGUA (Culiacan DGE station) and the climatological station at the Biology Faculty of the UAS. In this case study, the latter was considered since it is located closer to the basin under study. 2.2.5. Statistical Analysis and Probability in Hydrology  
When designing a storm drainage system, as well as other infrastructure systems, estimations of future events are needed such as the maximum precipitation in a time interval or the runoff volumes accumulated in a time interval [36]. The estimation of a future phenomenon is linked to a probability of occurrence. This is determined with a series of criteria, including the lifetime of the structure, the cost of the structure, possible human occupancy, and material damage in case of failure, among others [19]. The return period or recurrence interval is measured in years and is estimated depending on the relevance and risk of the hydraulic structure to be designed and built [33]. For this estimation, in Mexico, the values recommended by CONAGUA in the Basic Data for Potable Water and Sewerage Projects Manual are used. Similar recommendations exist in other countries [37]. When there is a hydrological data sample, such as rainfall, it is convenient to calculate its statistical parameters, which are representative values of the general characteristics of the data sample. In turn, they define the characteristics of a population, for which, there are equations that allow for an evaluation of both the statistical parameters of the sample and those of the population [21].