Evaluation of the perspectives of sustainability and resilience of the two agrosystems with sorghum and beans in the study area

The evaluation of the effect of management practices for sorghum and beans in the two rainfed production systems to decide whether or not they are sustainable and resilient was necessary to use the long-term simulation scenarios (100 years). This evaluation was based on three main criteria: the productive life index (IVP), the productive loss index (IPP) and the equilibrium yield (RE), IVP and IPP are supported by the erosion/productivity index (IEP), concept introduced by ^{Perrens and Foster (1984)}; ^{Williams et al. (1984)} and RE is the scope of performance variation, which are described below.

Erosion-productivity index (IEP)

This is defined as the relationship between the annual yield of a crop on eroded land and the annual yield of the same crop on a non-eroded land.

Productive life index (IVP)

This index, defined as the period of time to which a production system ceases to be sustainable and vulnerable by falling below a certain level of productivity (in this case IEP< 0.4 for the sorghum crop and 0.6 for beans, respectively), was considered important in this evaluation, to determine the productivity trends of sorghum and beans.

Productive loss index (IPP)

The magnitude of the loss of productivity of sorghum and beans due to accumulated soil erosion within the 100-year planning horizon was assessed by means of this index. This was defined as the slope of the curve obtained by plotting the IEP on the Y axis against the accumulated erosion on the X axis.

The evaluation criterion in this case was that, if the system presents IPP values close to zero, it would be sustainable and resilient since it is indicative of insignificant soil losses of yield. Threshold values that determined this criterion are not defined, therefore, as long as they are not determined for the conditions of the study area, the criterion of the researcher to characterize them is fundamental.

Balance performance (RE)

A sustainable and resilient system implies an equilibrium production in which the annual variation of the yield oscillates within a small amplitude. The evaluation of this equilibrium level means quantifying the amplitude of variation of the performance in relation to a ‘typical’ range of fluctuation in the environment of the study area. Therefore, the RE defined as the deviation of the yield with respect to the average value in time, was used to consider the seasonal variations of the climate of the region and the effect of the accumulated erosion, and its effect on the productivity of sorghum and beans in the time scale.

Productivity of the sorghum-bean rotation in SAF

With the changes introduced in SAF (innovative system), it was possible to increase the productivity of sorghum and beans by 112 and 217%, exceeding the average commercial yield of 3 to 6.6 t ha^-1 of MS in sorghum and from 0.57 to 1.4 t ha^-1 in beans, respectively. Another difference between both systems is that the innovator proposes a crop production scheme different from conventional methods and an alternative productive soil management. Its contributions are significant in the increase of production, water retention, reduction of runoff and the production of sediments. However, beyond the productive criterion, two features of this system have an enormous ecological value: their capacity to resist and recover from extreme climatic events to sustain themselves over time (Figure 1) and explanation of the above, their ability to consider the interactions of natural resources (soil, water, plants, microorganisms), without destroying or deteriorating them, so that their resilience increases.

Figure 1 Yield of the rotation sorghum (a) and beans (b) simulated by EPIC in Sandovales, Aguascalientes, for a period of 100 years (50 years old sorghum and 50 years old bean).  

In the traditional system (ST), a remarkable low productivity of the evaluated crops is observed, the yields of sorghum and beans in monoculture show a clear tendency to remain at a subsistence level with the passage of time, being more marked for sorghum, situation that is probably related to the tendency of runoff and erosion in the future as a result of using ST. This means that, under current production conditions, it is necessary to replace ST with a conservation one that reverses these trends and ensures greater resistance to climatic events, translating into less vulnerability and greater sustainability and long-term resilience, thus maintaining a high level of performance. sustained of the crops under these conditions of dry land production in the semiarid region. In addition, it can produce collateral benefits for subsistence and contribute to the reduction of poverty.

Variability of the productivity of sorghum and beans

With the integration of technological innovations, it is estimated that it is feasible to achieve an increase in the yield of sorghum and beans (Figure 1). The performance simulation analysis shows that the SAF productivity for the rotation sorghum and beans fluctuates less (with a CV= 30 and 16%) than the ST (CV= 32 and 47%). In both cases, this variation is probably due to differences in the amount and distribution of rains that occur between one year and another and during the growing season. However, even in the driest years, the production of the innovative system was higher (p≤ 0.05) than that of the traditional one.

Loss of soil and water for a 100-year scenario

In Figure 2, simulated runoff and erosion data are presented, with the EPIC model for sorghum and bean crops under the two production systems evaluated (SAF vs ST) for a period of 100 years, in which it is observed that the highest losses of soil and water were in the ST for both crops. The SAF significantly reduced runoff and erosion compared to the previous one. SAF, as expected, turned out to be a more efficient system to reduce runoff and erosion because, according to ^{Figueroa-Sandoval et al. (2011)}, in addition to having the efficiency of the stroke to the contour, modifies two factors that influence the surface runoff and erosion, the length and the degree of original terrain slope; as well as the positive effect of the live nopal barriers and the plant cover of annual crops (sowing of sorghum and beans at high plant densities).

Figure 2 Long-term simulation of runoff (a, b) and erosion (c, d) for the agroforestry system vs traditional. Sandovales, Aguascalientes.  

In general, the results obtained agree with what has been reported by other authors (^{Larose et al., 2004}; ^{Figueroa-Sandoval et al., 2011}; ^{Camas et al., 2012}) regarding comparative efficiencies, since the absolute values of the soil and water losses vary considerably according to the rational management of soil and crops. Because conservation practices are designed to rationally use the land and properly manage runoff (^{Lal, 1991}; ^{Farahani et al., 1998}). Reducing erosion and surface runoff are more evident and practically sustainable and resilient to the SAF’s hedgerows, achieving almost total erosion control and utilization of rainwater and life spans of 100 years, taking into account the rotation sorghum-beans (Figure 2). These results show that the current ST is not sustainable and is more vulnerable to erosion and runoff, so it is necessary to incorporate technological changes if we hope to achieve sustainable agriculture in the region studied.

Fractal analysis of the rain, runoff, erosion and yield time series of sorghum and beans in two agrosystems

Table 1 shows the values of the Hurst exponent (H) and the fractal dimension (D) calculated for each of the series of values of the response variables: annual precipitation, biomass and grain yield, runoff and erosion, corresponding to the Figures 1 and 2. In the case of rainfall for a horizon of 100 years, it favors the stabilization tendency of the annual variability of its behavior, since it presents a clear fractal pattern with the persistence of D= 1.299 and H= 0.701. With values of D close to 1, the process becomes more and more reliable, since it exposes greater persistence. That is, the tendency of the series of precipitation is strengthened as time passes. Like the rain, the bean yield series for the SAF (Figure 1b) according to the exponent H (0.651) can be established that the series is persistent therefore, the variable is sensitive to its history. The persistence process increases as the value approaches 1. However, the yield series of sorghum and beans for SAF_SO and ST_SO-FR, are antipersistent (0< H< 0.5); that is, the series tend to oscillate more erratically (Table 1).

The results coincide with previous studies by ^{Oñate, (1997)}; ^{Amaro et al. (2004)}; ^{Pérez et al. (2009)}. If 0<H<0.5 means that the time series is antipersistent. An antipersistent system covers less distance than a random one, in the case of an erratic particle. The values of H and D reported in Table 1, allow to determine that the runoff series (Q) for SAF_SO-FR are antipersistent; this means that, if in a small range of time Q it increases, the probability is high that in the following range of

time Q it will decrease. With a mean value of D= 1753, which indicates that the series of Q tend to oscillate more erratically. However, the Q series for ST_SO-FR, show long-term persistence. This means that this series is strengthened as time passes.

Finally, when considering the values of H and D for the erosion series (E), in Table 1 it is observed that for SAF with rotating sorghum and beans, it shows a high persistence. That is to say, the tendency of each series is strengthened as time passes. However, the erosion values of SAF in both crops are below the allowable ones. Thus, SAF increases its resilience against possible extreme weather events such as high intensity storms.

The results confirm that the annual precipitation at the Sandovales site, Aguascalientes, can be characterized through its fractal dimension, reaching values of D close to 1 and a high H exponent (0.701), which corroborate a high level of persistence. On the other hand, the results that show the majority of the series of values of the response variables in the management systems evaluated (SAF_SO-FR vs ST_SO-FR) under rainfed conditions corroborate that these phenomena can be characterized through their fractal dimension. In practical terms, the analysis contributes to the estimation of hydrological phenomena associated with the variability of precipitation and the design of ‘systems’ aimed at modifying production techniques in order to sustainably and ‘resiliently’ increase productivity and overall profitability of the traditional productive systems.

Through fractal analysis, the temporal variability of a time series and its persistence can be determined. Likewise, the existence of a change in the pattern of precipitation and the different phenomena associated with it can be determined according to historical evidence.

Perspectives of sustainability and resilience of the productivity of the SAF with unirrigated dry-land sorghum-beans

In the Table 2 shows the values of the three evaluation criteria for sustainability and resilience of sorghum and beans for the two agrosystems compared. According to the value of the IVP, the ST with sorghum is not productive (the IEP goes below the minimum value of 0.4) practically from the first years of continuous cultivation (IVP= 0.02). On the other hand, ST with beans presents an IVP of 0.8 (IEP greater than the value of 0.6) which indicates a productive life of practically 80 years and finally, the SAF with rotating sorghum-beans has an IEP value of 1, which indicates that, in the 100-year evaluation period, the SAF is sustainable and resilient. From the results, it is inferred that, under this criterion, the rotation of sorghum-bean is kept closer to its optimum conditions, due to agricultural practices and soil conservation and water harvesting that integrates the SAF.

In relation to the IPP, Table 2 shows that ST with sorghum has the highest slope of the IEP curve of the two agrosystems considered, this is an indicator that this production system has a yield loss of 0.54. t of dry matter in 100 years of continuous cultivation that equals a loss of 100% of productivity. On the other hand, the ST with beans presents a relatively low IPP, with this system, the beans lose 0.1 t of grain in 100 years, which is equivalent to a loss of productivity of 20%. Finally, the SAF presented a very low IPP, the sorghum-bean rotation loses a negligible amount of its productivity. Therefore, the results obtained for the SAF with this crop rotation exhibit a greater stability in yields and lower productivity reduction in drought conditions, unlike monocultures.

Finally, when considering the RE criterion, in Table 2 it is observed that for ST with sorghum, the scope of variation of the IEP gave a standard deviation of 0.53, the ST with bean of 0.1, SAF with sorghum of 2.14 and with beans of 0.68, respectively. In this case a hypothesis test of non-significant equality of variance, which meant that the scope of variation was the same in both production systems for the two crops.

Based on the results observed in this evaluation it is clear that, of the two production systems analyzed, SAF is the one that presents the best perspectives to resist or recover from extreme climates (droughts or intense rains). Simultaneously, it allows to achieve a sustained and resilient yield of sorghum and beans under crop rotation, while the ST, currently used, it is necessary to replace it to avoid further deterioration of the future productivity of the system in the study area, especially the ST with monoculture.