4.1. Patterns of the climatic information obtained
As already known, temperature shows some patterns, like its decrease related to the a higher topographic altitude [
41] or higher values during the summer season or a drop at higher geographical latitudes [
42,
43]. The analysis of the temperature data included herein shows the common patterns of temperature variation based on altitude; however, an analysis per season shows that the maximum temperature, unlike the minimum (with high values during the summer and low values during the winter), in the Andean areas (> 1500 m.a.s.l.) takes place in September, October, and November. The aforementioned pattern for maximum temperature was also observed by [
44], also indicating that the month with the highest values is November. On the other hand, the reason why September, October, and November are the warmer months in the Andean area would be related to seasonal rainfall. The summer months (including part of December) in the Andean area correspond to the rainy season; therefore, the presence of cloudy masses is much more frequent, thus limiting solar radiation towards the ground surface. This “shadow” created by the clouds, together with the winds created by low pressure, would be responsible for lower maximum temperatures during the summer months. It is worth noting that the variation of the maximum temperature in the Andean areas is not very sharp throughout the year. Like [
42], this research shows that this variation is around 2 °C. As for the temperature and longitude relation, as [
42] and [
45] indicate, it decreases from north to south, but the maximum temperature is sometimes contradictory. In these cases, the location of the station is worth noting; e.g., if it is located in plains or inside valleys. Topographic features model some temperature and precipitation characteristics [
13], which, in turn, would potentially interact with wind speed and direction. In the case of valleys, a higher temperature could be caused by the limited movement of air masses.
Precipitation in our environment determines seasons throughout the year, i.e., there are only two seasons: Wet and dry [
27,
30,
31]. As for patterns of precipitation, the latter seems to be related to altitude; however, it is possible to find a certain idiosyncrasy in it [
12]. The general precipitation patterns for the western Andes of Peru indicate that precipitation increases with altitude [
30] and decreases from north to south [
27]; here, however, we will analyze the coastal and Andean areas, specifically. The coastal area of Peru is influenced by three factors defining its wet season: The Humboldt current, the trade winds and the thermal inversion layer in the lower troposphere [
30,
32], which, during the austral winter, produce very fine rain known as drizzle or misty rain that comes from the stratocumulus clouds and does not go higher than 1000 m.a.s.l. [
30,
33]. Based on what was obtained, precipitation in the stations located close to the coast is scarce and it increases in those located further inland, where there are hill ecosystems (precipitation occurs even inside the valleys). Probably, the increased precipitation is caused by the clouds “compressing” and producing rain when bumping into the western areas of the coastal mountain range. It is worth noting that the regular wet periods occur during winter. Nevertheless, in December, January, and February there is also a fair amount of precipitation that is due to ENSO events, when the Humboldt current debilitates and, therefore, the ocean becomes warmer, causing heavy rains [
32]. As for the Andes, the rainy season is quite noticeable, being limited to summer. Rainfall in the Andean area is produced by geographic and physical factors, the most outstanding of which are the Intertropical Convergence Zone (ITCZ) and the Amazon basin [
27,
30], which regulate precipitation, decreasing from north to south and from east to west (the latter being related to altitude). All the aforementioned patterns are visible for the area of study, where the higher the elevation, the greater the amount of precipitation (the reference being the constant geographic longitude), while the northern area of Arequipa receives the most precipitation, unlike Tacna (this, considering the same altitude).
Precipitation in our environment determines seasons throughout the year, i.e., there are only two seasons: Wet and dry [
42,
45,
46]. As for patterns of precipitation, the latter seems to be related to altitude; however, it is possible to find a certain idiosyncrasy in it [
19]. The general precipitation patterns for the western Andes of Peru indicate that precipitation increases with altitude [
45] and decreases from north to south [
42]; here, however, we will analyze the coastal and Andean areas, specifically. The coastal area of Peru is influenced by three factors defining its wet season: The Humboldt current, the trade winds and the thermal inversion layer in the lower troposphere [
45,
47], which, during the austral winter, produce very fine rain known as drizzle or misty rain that comes from the stratocumulus clouds and does not go higher than 1000 m.a.s.l. [
45,
48]. Based on what was obtained, precipitation in the stations located close to the coast is scarce and it increases in those located further inland, where there are lomas ecosystems (precipitation occurs even inside the valleys). Probably, the increased precipitation is caused by the clouds “compressing” and producing rain when bumping into the western areas of the coastal mountain range. It is worth noting that the regular wet periods occur during winter. Nevertheless, in December, January, and February there is also a fair amount of precipitation that is due to ENSO events, when the Humboldt current debilitates and, therefore, the ocean becomes warmer, causing heavy rains [
47]. As for the Andes, the rainy season is quite noticeable, being limited to summer. Rainfall in the Andean area is produced by geographic and physical factors, the most outstanding of which are the Intertropical Convergence Zone (ITCZ) and the Amazon basin [
42,
45], which regulate precipitation, decreasing from north to south and from east to west (the latter being related to altitude). All the aforementioned patterns are visible for the area of study, where the higher the elevation, the greater the amount of precipitation (the reference being the constant geographic longitude), while the northern area of Arequipa receives the most precipitation, unlike Tacna (this, considering the same altitude).
4.2. Modeling of climatic surfaces and validation
An important aspect to be taken into consideration in modeling is the representativeness of the area, i.e., the quality, quantity and, in this case, the correct geographical position of the input data to perform the interpolations. Some of the produced models use only information from weather stations, satellites or a combination of both [
12,
14,
15,
19,
20,
24]. In tropical countries, the problem of the lack of climate information, either due to the lack of meteorological stations, to the difficult access to information or to incomplete records is highlighted [
15,
27,
28,
48]. In this sense, it is possible that the models produced by other authors for the area of study of this research will not properly reflect the climate reality, because only a few ground stations were used or because data were mainly collected by satellite [
14,
15,
19,
20], with biases related to topographic complexity and to the density of meteorological stations [
27,
48]. For this case, the use of as much ground-based climate data available, considering the recommendations by Cuervo-Robayo [
24], allowed to obtain “truer” surfaces, better reflecting the temperature and precipitation patterns. It should be noted that the coastal area, unlike the Andean area, has fewer meteorological stations (27% of the total) and that most of them are located near the coast. It is known that the coastal area of Peru has peculiar weather characteristics, with drizzle during the winter, which intensifies inland, towards the windward side of the lomas, creating the lomas ecosystems [
42,
49]. As previously indicated, the meteorological stations for this area are close to the coast and only two are close to the lomas ecosystems (Atiquipa station, in Arequipa, and Sama Grande, in Tacna).
Given the peculiarities of the
lomas ecosystems, and in order to better represent them in the bioclimatic models, it was decided to produce “virtual” stations in the sense that they would have correlated information between the real weather stations (Atiquipa and Sama Grande) and the NDVI index, somehow covering the information “gaps”. The NDVI index, as such, represents health and vegetation cover; therefore, in natural situations, this index is mainly influenced by the amount of moisture in the soil [
50,
51] and, therefore, of precipitation. In fact, the NDVI-precipitation relation has already been written about, making it clear that they are closely correlated [
52,
53,
54]; although temperature is also related [
55,
56,
57], it seems that, in places where thermal fluctuation is not very wide, this variable does not play a preponderant role [
57]. On the other hand, it is also mentioned that the NDVI response, timewise, depends on the nature of rainfall [
57]. Thus, in areas where the wet season is sudden, there will be a lack of synchronization between precipitation and NDVI. In turn, in places with a progressive increase of humidity, NDVI peaks and precipitation will be quite close. It should also be noted that, in areas with a marked seasonality, the NDVI-precipitation relation is also close [
57]. Then, taking into account the characteristics of precipitation in the formations known as Lomas, it evidently is the marked seasonality and a progressive appearance of vegetation [
30,
49,
58]. In this research, apparently the “virtual” stations created based on the NDVI and precipitation values from real stations was satisfactory, since it shows very significant differences compared to other models. Also, the areas predicted by the model almost perfectly match the
lomas vegetation maps [
29,
30]. In fact, the results show how precipitation occurs in certain sectors of the coastal mountain range (at a certain altitude, slope and orientation), recalling that these ecosystems are specific oases of life surrounded by one of the driest deserts in the world [
59]. Anyhow, this is just an applicable solution for these particular ecosystems with closely related climate and a vegetation [
60]. Although the performed mathematical operations seem simple, they are a way towards a solution regarding lack of information in the
lomas ecosystems. Though in the coastal area of southern Peru there are weather stations which are very close to the coast, these do not reflect the precipitation occurring a few kilometers inland, in the lomas ecosystems. Even during the ENSO (El Niño-Southern Oscillation) events, in stations located close to the coast, precipitation is considerably lower compared to that in the lomas [
61]. The veracity of the values found for the “virtual” stations could not be verified here, since the only two stations present in these ecosystems were used for data generation. However, the intention here is that the precipitation conditions in these unique ecosystems are adequately reflected, since other global models only show “flat” values or precipitation mostly restricted to the coastal valleys. The
lomas ecosystems are unique in the world, the greatest number of endemic species being found towards the south of Peru [
62]. Thus, for a good representation of the niche and distribution models of the species, it is necessary to show them as close as possible to what happens in reality. Further studies are needed to understand the relationship between precipitation and NDVI in
lomas ecosystems.
The use of geographic (latitude and longitude) and topographic covariates is common in the modeling of climatic surfaces [
14,
15,
24]. Nevertheless, some others also consider the use of other covariates, such as the cloud cover or the distance from ocean coastline to inland [
13,
14,
63]. Generally, the most used covariate in modeling is altitude [
14,
15,
24] and it is closely related to temperature variations [
64]. On the other hand, precipitation, in very broad terms, can show a relation with altitude [
45]. However, locally, it is idiosyncratic [
19] and will depend on other factors, such as wind currents, topography and the diurnal cycle of solar radiation, these being the most complex in mountain areas [
13,
28]. In this case, as recommended by some authors [
13,
24] and considering the topographic complexity, slope, land orientation, TWI, distance to the coast, and cloud cover, were used for modeling, besides altitude, producing good results. In fact, towards the coastal area, where other surfaces show a poor representativeness of precipitation in
lomas ecosystems, here they are clearly shown to occur towards the western slopes of the hills (windward).
The use of parameters to measure errors in models is common and necessary, because these parameters indicate de degree of bias or the difference between data collected on site and data interpolated by some model [
65,
66]. On the other hand, the use of validation values is common to verify the interpolated results in climatic layers with real values collected on site [
12,
14,
24,
67]. In this case, a k-fold cross-validation was used, since the reduced amount of available data made it difficult to take a certain percentage for a single training and test [
68], which is why it was decided to perform 10-folds with random data and produce models with each of the “sets” of data obtained, finally evaluating the surfaces obtained through the RMSEcv. According to RMSEcv and MAD, the precision of the produced models is adequate and presents relatively low values. Nevertheless, there are some considerations regarding the interpolated variables. Regarding temperature, the RMSEcv and MAD values are higher for the maximum temperature, which indicates more variability, and this is reasonable since the maximum temperature could be influenced by different factors (as explained above), while the minimum temperature is more homogeneous and responds to seasonality [
42]. Precipitation, unlike temperature (maximum and minimum) has slightly higher values for RMSEcv and MAD, a pattern also observed in [
14,
24]. As mentioned before, precipitation is idiosyncratic, with different patterns according to the combination of different factors [
28]. This is why the variability of the precipitation models is somewhat high, even more so in places with a complex topography and a poor number of meteorological stations [
14]. In this sense, probably for this study, integrating the use of covariates related to topography, humidity and the use of the largest amount of data from meteorological stations favored the low RMSEcv and MAD values. Finally, it is worth noting that the higher RMSEcv and MAD values related to precipitation were obtained in the months of the wet season in the coast (winter) and the Andes (summer), showing once more that it is during these periods when the most variations take place according to the precipitation patterns of each place.
4.4. Expectations about the use of bioclimatic layers
The purpose of the produced 19 bioclimatic layers is to provide support for the conservation of flora and fauna species distributed in the departments of Arequipa, Moquegua, and Tacna. Having data that are more representative of the climatic reality of this area will guarantee the production of ecological niche models closer to reality, better outlining the environmental requirements of the potentially studied species, and this not only for endemic species, but also for “problem” species (for example, introduced or parasitic species), or even to detect evolutionary relationships, through the concept of niche conservatism. On the other hand, the surfaces produced here could also be used, for instance, to evaluate the historical biogeography of the species or how climate change could affect them (this, through models projected to the past or to the future, considering the processes or methodologies applied to obtain them). The modeling of the ecological niche (ENM) of the species with layers that are more similar to the climatic reality will allow us to better know the ideal environmental conditions for the species, and, with this, after its projection in the geographic space, we will be able to obtain, in a more approximate way, its geographic and potential distribution (SDM). Finally, it is expected that his work is a source of inspiration to better recreate the environmental and bioclimatic space of Peru, since, as we know, our territory, having a complex topography, needs on-site data for a better climatic representation of its surface and, thus, better understand the ecoclimatic requirements of its species.