Water erosion

The current water erosion was estimated using the Revised Universal Soil Loss Equation (RUSLE)⁹:

Where:

E= soil loss per surface area unit (t/ha/yr);

R= rain erosivity factor (MJ mm ha^-1 h^-yr^-1);

K= soil erodibility factor (t ha^-1 h^-1/MJ mm ha^-1);

L= slope length factor (non-dimensional);

S= slope degree factor (non-dimensional);

C= vegetation factor (non-dimensional);

P= mechanical practice factor of erosion control (non-dimensional).

The IDRISI Selva software¹⁰ was used to measure it; this software includes the RUSLE command, which is a routine that improves the calculation of the L and S factors and including other additional routines that eliminates depressions, and well as others that allow a better generalization of the digital elevation model. The isoerosivity map, which divides the country into 14 regions, was used for estimating the R factor; the state of Zacatecas is located in regions 3, 4 and 7, whose equations are the following:

Where Y is the R factor in terms of MJ mm/ha h, and X is the annual precipitation in mm.

The values of the K parameter were estimated according to a table of erodibility values developed by Figueroa ¹¹. The K factor is a function of the soil texture and class. These values were assigned to soil classes and textures within ejido lands, which were digitized to facilitate their management within geographic information systems (GIS), through the IDRISI Selva software¹⁰. The output is the soil loss for each vegetation type.

The effect of climate change was quantified using the information system of INIFAP for the Mexican Republic ¹², which consists of the climate data base of the 1961-2010 period and the climate scenarios predicted for the years 2021 to 2080 in the Representative Concentration Routes (RCP) 4.5 and 8.5 of greenhouse gases (GHG) for, among others, an ensemble model formed from 11 general circulation models (GCM) ―reduced in scale and calibrated¹³― selected for Mexico (BCC ‐ CSM1‐ 1, CCSM4, GISS ‐ E2 ‐ R, HadGEM2 ‐ AO, HadGEM2 ‐ ES, IPSLCM5A ‐ LR, MIROC ‐ ESM ‐ CHEM, MIROC ‐ ESM, MIROC5, MRI ‐ CGCM3, NorESM1 ‐ M).

The values of maximum temperature, minimum temperature and monthly precipitation were used for the years 2021 to 2080. The annual data for the climatic scenarios 2021-2040, 2041-2060 and 2061-2080, hereinafter referred to as climatologies or 2030, 2050 and 2070 years, respectively, were estimated based on the monthly data.

The information of INEGI ¹⁴ on land use made it possible to divide the effects of rainfall and water erosion by predominant vegetation type, including grasslands as well as areas covered with other types of vegetation such as scrubs and forests, and agricultural land. With the information of water erosion estimated for the present and for the years 2030, 2050 and 2070, regression models¹⁵ were obtained for the main vegetation types of the state, mentioned above, calculating an annual rate of change for each condition of vegetation and comparing it with that of the grassland area, in order to determine where the greatest impacts due to climate change will potentially occur.

Comparison with other vegetation types

The effect of the decrease in precipitation on the reduction of water erosion of the soil in the grasslands of Zacatecas is different from that of other types of vegetation in the state; assessment of the impact on the reduction of rainfall in the scrubland, forest and agricultural areas shows that the estimated values for the scrubland area are the lowest (Table 3), although the pattern is similar to that projected for the grasslands. Table 3 lists the estimated precipitation values for each type of vegetation and the percentage reduction. The reduction values are lower for RCP 4.5 than for RCP 8.5, because the former is associated with a lower effect of greenhouse gases (GHG). The reduction in precipitation values affects the average erosion values of the different types of vegetation studied.

Tables 5 and 6 show the reduction in mean values of water erosion estimated using RCP 4.5 and RCP 8.5. Table 5 shows a decrease in percentage of 5 to 9 % of current values (2010); this is most prominent in the grasslands, where the values range from 16.13 to 15.18 t/ha/yr, amounting to a 5.89 % reduction in 60 yr and showing the lowest values among all vegetation types. However, this projection may be altered by changes in land use. Grazing is carried out in this area, which is thereby subject to risks of deterioration, especially when overgrazing occurs¹⁶; this has been recognized as one of the main causes of increased runoff and water erosion, since, by reducing the vegetation cover, infiltration is reduced and runoff increases¹⁷. The decrease in the vegetation cover can also be associated with the reduction of precipitation. Long periods of drought have been recognized as causing and contributing to the increase in erosion. The risk that the change of land use entails for the grasslands has been shown in the reports of land use change rates as they were converted to irrigation agriculture areas during the 1970s, in what was formerly the grassland area within the “Chupaderos” aquifer in the state of Zacatecas, where the said rates were up to 1,913 ha/yr¹⁸ and 678 ha/yr in the Aguanaval aquifer¹⁸. A change in land use can induce changes in water erosion of a grassland area to erosion values typical of the agricultural area, whose average values range from 133 to 161 t/ha/yr.

Likewise, it can be observed that average values of water erosion will decrease in the scrubland area and that they range from 5.18 to 11.29 %, which is related to the decrease in rainfall forecast for the coming years and whose exchange rates are significant (P<0.05). As in grasslands, overgrazing and changing land use are the greatest risks that this decrease in water erosion may not be as predicted. Another risk for this area is wind erosion, since it is in this type of vegetation that the lowest rainfall in the entire state will occur, which may favor the increase in wind erosion associated with drought^19,20. A situation similar to the previous two will be observed in the forest area, where reductions in water erosion from 6.86 to 17.4 % are expected, without the exchange rates being significant (P>0.05). In this vegetation type, the felling of trees or soil change, as well as the effect of drought and diseases on trees, which have been found recently, can be seen as the greatest risks²¹.

Finally, mention is made of the case of the agricultural lands, which, since they are almost always located in the lowest part of the basin, are subject to runoff of greater magnitude and greater erosion. They are also the sites with the greatest depth of soil, which renders them the areas with the largest amount of soil losses. However, given the prospect of decreasing precipitation, it is estimated that water erosion values will decrease from 9.81 to 17.48 %, this area being the most benefited in terms of decreasing water erosion, with lower exchange rates (P>0.05). The appropriate management of the soil resource, the management of slopes, tillage techniques for soil conservation and maintenance of plant cover are technological components that contribute to improving soil care. However, traditional soil management practices, furrows in the direction of the slope and tillage during periods of intense winds, can increase the values of water and wind erosion. The latter has been reported¹¹ as an erosion risk equivalent to that caused by runoff. The establishment of windbreak barriers and changes in tillage and moisture conservation techniques should be a priority to maintain agricultural productivity and not increase water erosion values, as predicted by the models used here. Additionally, attention should be paid to the reduction of authorizations for land use change, since, as mentioned above, the change in land use can lead to soil loss levels equivalent to those occurring in the agricultural area.

On the other hand, although it seems that the most notorious effects of land losses in Zacatecas, occur in the agricultural areas, land losses in other vegetation types are as much or more harmful than in the agricultural areas, because soil depth in grassland, forest or scrubland areas, are much smaller than in agricultural lands. Therefore, a decrease of one centimeter of surface layer of soil causes more dramatic effects in those other areas than in agricultural lands.

As mentioned at the beginning of this section in regard to soil losses due to water erosion, unlike other indicators, water erosion will tend to decrease as the global warming effect due to accumulation of gases in the atmosphere reduces precipitations in the state of Zacatecas. Results show that it is important to have information that may allow planning in the medium and long term, as well as expanding and including studies on related issues, such as wind erosion and other types of degradation associated with global warming, and other studies that may contribute to complete the understanding of the changes that will occur in the future and determine preventive actions and actions to care of the soil resource.