1. Introduction
Olive grove landscapes form socioecological systems characteristic of Mediterranean environments [
1,
2]. They are particularly representative in Spain, covering more than 2.5 M ha [
3]. In Andalusia, these olive grove landscapes, with an area of 1.5 M ha, form a multifunctional system making an essential contribution of ecosystem services to society, highlighting the production of olives and olive oil. During the last five growing seasons (2012/2013–2016/2017), the region produced an average of 1.19 M t year
−1 of olive oil [
4,
5,
6]. In addition, as with many agricultural matrix landscapes with a heterogeneity of habitats and ecotones, olive groves have been shown to serve as reservoirs of biodiversity [
7,
8].
The enforcement of the Common Agricultural Policy (CAP) in the European Union in 1962 led some traditional farming systems (mountain farming and agroforestry systems) to collapse due to their lack of adaptation to the new regulations (levels and forms of production). These systems were based on the optimisation of the use of natural resources and were adapted to local climatic and geomorphological conditions, but their production levels were relatively low, and they were not competitive with the environmental and sanitary standards established by the CAP or market prices [
9]. Faced with this situation, farmers’ decisions showed two opposing trends: abandoning their lands in the face of declining economic benefits or intensifying (i.e., increasing or introducing) the use of synthetic agrochemical herbicides, pesticides or fertilisers; generating higher plant density and adopting technological improvements [
10]. Both intensification and abandonment of farming systems can have undesirable environmental and socioeconomic consequences. On the one hand, agricultural intensification improves productivity (increasing economic benefits) but is generally accompanied by environmental damage [
11], such as soil erosion [
12,
13,
14] and spatial homogenisation (expansion of monocultures), which affects biodiversity [
15]. On the other hand, the abandonment of agricultural activities leads to a total loss of profitability and the degradation of social stability [
16], while at the same time causing the accumulation of biomass (scrubbing) fuel by ecological succession (passive restoration), increasing the risk of wildfires [
17].
Olive grove agroecosystems have traditionally formed agricultural landscapes [
18], as they are adapted to Mediterranean climates where periods of summer drought and water stress are inherent to their dynamics [
19]. However, in the last 50 years, the expansion of irrigation to these agroecosystems has increased for merely productive purposes [
20,
21]. Irrigated olive groves in Andalusia have grown from 5% to about 35% of the total agricultural area [
22,
23]. Despite the resistance of olive trees to drought, irrigation is important to ensure adequate yields in years with little rainfall in order to reduce the variability of yields from year to year because of the alternate bearing and to increase olive oil production. Despite the high delivery efficiency of some irrigation systems (i.e., drip irrigation), there is high risk of water overuse. This, in turn, could impact the demand for water for human consumption in certain basins associated with large extensions of intensive olive groves and exacerbate soil erosion [
24]. Specifically, the average values of soil loss were 19 t ha
−1 year
−1 in the 1950s but are highly variable at present, ranging from 23 to 184 t ha
−1 year
−1 [
25,
26]. To reduce erosion and prevent soil degradation or contamination, various conservation techniques, such as vegetation cover or terracing, are applied. However, a soil water balance adapted to the needs of the farming system is also important and could be achieved by increasing water infiltration, avoiding soil compaction and evaporation. In addition, to achieve this balance, it is essential to apply the appropriate irrigation system.
The impacts of the intensification of management highlight the need for policies/strategies designed ad hoc for olive grove agricultural landscapes with the objective of promoting the adequate conservation of the soil, maximising the viability of these agricultural systems and delaying the advance of erosive processes and their consequences [
27,
28,
29]. Although several factors and management practices other than irrigation can have multiple consequences on the soil environment (i.e., agriculture mechanisation), there are few studies that quantify the consequences of erosion associated with irrigation on the soil and its impact on long-term economic profitability. Considering the restrictions that climate change will impose on the availability of water resources in the near future [
30,
31], and with the biophysical system serving as the foundation on which any agroecosystem is based [
32], it is urgent to study the relationship between erosion, irrigation and profitability in the agricultural landscapes of olive groves. Thus, the objectives of this work, using a case study of an olive-growing region in Andalusia, were (a) to characterise the soils of the studied region considering different erosive levels and olive-growing management systems and (b) to compare, by means of medium- and long-term time projections for different erosive levels, the possible productive and economic consequences for irrigated and rainfed olive groves [
33]. The use of time simulation models is a useful and valid tool to explore the uncertainty about the future consequences that may result due to the factors considered in this study (erosion, production and irrigation). Finally, the sustainability of the olive grove landscapes was evaluated based on the soil properties and profitability analysed.
3. Results
All soil parameters analysed by PCA showed strong collinearity (determining value < 0.001) and all parameters showed a normal distribution and homoscedastic behaviour, except for the soil gravel content (G) and moisture. The interactions between olive-growing management and erosion were highly significant (p < 0.001 ***) for all dependent variables, thus leading to a separate analysis of both factors.
3.1. Soil Characterisation of the Olive Grove
3.1.1. Soil Characteristics and Water Management Regimes Considering Coincident Levels of Erosion
Table 3 shows the main results regarding the descriptive statistics and significant differences for the physical soil parameters analysed in the coincident erosive levels (i.e., moderate and severe) in rainfed and irrigated olive groves in the
Estepa region.
Significant differences were found in the gravel content and moisture of the soil. Irrigated olive crops presented greater soil moisture (36.79%) than rainfed crops. In turn, decreases of up to 90.81% in the gravel content and 4.85% in the soil pores were observed in the irrigated olive groves, resulting in a more compacted soil. Irrigation did not significantly affect the finest soil particles and, therefore, their overall texture.
Table 4 shows the results for the chemical soil parameters analysed.
Irrigation significantly influenced the concentration of nitrates in soils, increasing their concentration by 21.72%. At the same time, it resulted in a 19.90% decrease in organic matter but did not significantly affect the other chemical parameters considered.
3.1.2. Soil Characteristics and Water Management Regimes Considering All Levels of Erosion
Table 5 shows the soil physical parameters for the different erosive levels in each management type of the olive groves sampled. There were highly significant differences for each dependent variable (i.e., physical parameters) in at least one of the evaluated treatments (i.e., levels of erosion) (
p < 0.001 *** from the ANOVA and Kruskal–Wallis test). For each level evaluated in each variable, the results of the post hoc analysis through the realisation of a homogeneous subset matrix between the sampled treatments (i.e., null, slight, moderate and severe erosion under rainfed management, and moderate and severe erosion under irrigation) are represented. Two levels present different group classifications only when the results of the post hoc test suggested the existence of significant differences (
p < 0.05 *).
For the granulometric variables, a decrease in the gravel content of the soil was detected (particles between 2 mm and 6 cm) as the level of erosion increased, with this loss of gravel being more pronounced in irrigated olive groves. In this sense, rainfed plots with severe erosion together with plots with moderate and severe erosion with irrigation formed a single statistical group differentiated from the rest of the plots due to their low gravel content.
The estimated texture, closely linked to the limestone content characteristic of the study area, corresponded to soils of medium-fine texture with good water retention. Thus, the soil of Estepa was predominantly loamy, combined mostly with sands or silts, for all erosion levels and the two types of management, with significant differences between all treatments for sand and silt content. However, the irrigated olive groves showed a higher clay content in cases of moderate erosion but not in those of severe erosion.
Regarding the porosity, in the rainfed olive groves, this parameter decreased by about 12% as the level of erosion increased. In the case of irrigated olive groves, this decrease was even more accentuated: 2.33% and 7.39% in olive agroecosystems with moderate and severe erosion, respectively. As expected, soil moisture was significantly higher in irrigated plots (up to 65.53% more in moderate erosion plots and 53.13% in severe erosion plots). Groups with significant differences in porosity and moisture were established according to different levels of erosion (increasing with the degree of erosion), maximising these differences in irrigated olive groves. Ultimately, in rainfed plots, net losses of soil weight of up to 41.04% were reached, increasing this loss with irrigation; 15.13% in olive groves with moderate erosion and 11.17% with severe erosion. The slight, moderate and severe erosion levels in the rainfed management cases together with the level of severe erosion under irrigation did not show significant differences for the weight of the soil per unit area.
Table 6 shows the chemical parameters, indirectly indicating fertility, enzymatic activity and soil contamination, and classification results obtained by means of a Tukey post hoc test. A decrease in pH was observed (almost 8%) as the degree of erosion increased, while in the irrigated olive groves, slightly lower values were observed in the coinciding erosion levels, that is, moderate and severe erosion (0.24% and 0.63%, respectively). These two erosive levels formed a differentiated group with respect to rainfed management. For the parameter organic matter, lower values were detected in olive groves with high erosion (decreases of up to 62.43%). In irrigated cases, a decrease of 18.11% was detected in olive groves with moderate erosion and 23.74% in those with severe erosion, with significant differences observed for all treatments.
The enzyme phosphatase, which plays an essential role in the mineralisation of organic P and as an indicator of soil enzymatic activity, showed an increase of more than 100% between the lowest and highest levels of erosion. However, irrigation did not influence the presence of this enzyme, which was practically the same for comparable erosive levels. The concentration of nitrates, which are indicators of fertiliser use, increased directly with erosion levels up to almost 75% in rainfed olive groves. On the other hand, irrigation increased nitrate concentration by 25% and almost 30% in moderate and severe erosive levels, respectively.
3.2. Time Projection of Profitability in Rainfed and Irrigated Management
Table 7 shows the production and economic data obtained from surveys of farmers. These data correspond to each of the integrated management types considered: rainfed (
n = 16) and deficit irrigation (
n = 8). The responses of the farmers indicated that the decision to apply irrigation was related to the objective of increasing crop production, above any other consideration (for example, cost of implantation or related erosion problems).
On average, the production of olive groves is considerably higher under irrigation, with an annual production of 4249.50 kg of olives ha−1 (807.89 L of oil ha−1), compared with 2749.50 kg of olives ha−1 (522.72 L of oil ha−1) produced under rainfed management. In the olive groves with higher production per hectare, the estimated average production per tree is lower than in plots with lower production. This is due to the higher density of plants present in the olive groves of high production per unit area, where the trees are smaller and, therefore, their productive level is lower. In economic terms, the annual benefit (difference between sales revenue and costs) of 1 ha with an irrigated olive grove was 682.65 € ha−1, which increased to 1182.65 € ha−1 when considering CAP subsidies. In the rainfed plots, an annual profit of 922.60 € ha−1 was observed, increasing to 1322.60 € ha−1 when including the CAP subsidies.
Figure 3 shows the time projection of production and benefits per hectare of rainfed and irrigated olive groves, considering their levels of erosion.
In the olive groves with null, slight and moderate erosion, under rainfed management, the production remained relatively constant (with a small decrease with slight and moderate erosion), between 3000 and 2500 kg of olives ha
−1 year
−1 for the period considered (150 years). In the olive groves with moderate erosion, the production of irrigated olive groves was higher than that of unirrigated olive groves (4250 and 2565 kg of olives ha
−1 year
−1, respectively), maintaining a slight decline during the simulation time. The olive groves with severe erosion showed better production in irrigated management compared with unirrigated groves (3700 and 2200 kg of olives ha
−1 year
−1, respectively, in the first year of the period considered). However, in an interval of 100 years for rainfed olive groves and 145 years for irrigated olive groves, those with severe erosion would be abandoned due to a decrease in productivity below 1500 kg of olives ha
−1 (following the criterion of Gómez-Calero [
22]).
In economic terms, with moderate erosion, the profitability of an olive grove, either rainfed or irrigated, was close to 750 € ha−1. In rainfed olive groves, the value was practically constant around this value during the study period, while in the case of irrigation, it fell from a value of 700 to around 600 € ha−1 in the last year of the study period. With severe erosion, in the olive groves without irrigation, the estimated profitability was 500 € ha−1, while the irrigated olive groves presented a value of profitability of 250 € ha−1. Profitability would disappear at 100 years under rainfed management and 44 years for irrigated olive groves because the operating costs of the latter would exceed the minimum income from sales considered profitable (3263.28 kg of olive ha−1). With slight or no erosion, rainfed olive groves were profitable throughout the study period, with profits of 760 and 900 € ha−1, respectively. The CAP subsidies received by farmers obviously improve their profits but do not prevent the long-term loss of profitability of severely eroded olive groves, which must be abandoned after 90 and 40 years of simulation for rainfed and irrigated olive groves, respectively.
Table 8 shows the production and economic data per hectare accumulated throughout the simulation time. It was observed that soil erosion levels negatively affect production under both types of management in a highly significant way (
p < 0.001 ***). In the rainfed olive groves, severe erosion presented production values markedly lower than other levels of erosion. Logically, the decrease in production due to erosion significantly impacted the profitability of olive groves, with this significant relationship increasing in irrigated olive groves despite their increased production due to increased farm costs.
5. Conclusions
The results showed that irrigation in the olive agroecosystems in Estepa considerably increases the level of production immediately and in the short term but negatively affects their ecological and economic sustainability due to the degradation of the different soil parameters studied. The generalised irrigation type in the study area is localised and deficient in nature, with less environmental impact than other types of irrigation, such as sprinkler or blanket irrigation (also called flood or surface irrigation). Despite this, the results showed that, when comparing rainfed and irrigated olive groves with equivalent erosive levels, there were significant differences in some soil characteristics. The irrigated olive groves presented lower gravel content and higher soil moisture, decreasing their soil weight. Irrigation also gave rise to a loss of organic matter and a higher content of nitrates. Although irrigation increases production per hectare, this productive bonus may not be directly correlated with the economic benefits for farmers due to a lower selling price on the market for oils from irrigated olive groves and the cost of water. In this context, the promotion of deficit localised irrigation programmes would contribute to increasing water efficiency and saving, especially in situations of water scarcity, which are very common in Mediterranean olive grove areas.
We believe that the results of this study should be considered with some caution since the data analysed correspond to a single period and the absence of irrigation management in olive groves with null and slight erosion. Despite these limitations, the results show that management decisions must be taken in an integrated manner, considering not only economic–productive factors but also ecological aspects. This means that the notable expansion of irrigation in Andalusia should follow more rigorous guidelines considering the trade-offs that can occur with environmental aspects such as soil erosion. In any case, future lines of research should be oriented towards the optimisation of water yield [
78,
90], with the aim of improving the efficiency of this resource and increasing the general sustainability of olive groves. Exhaustive research should be carried out on the erosion–productivity relationship [
2,
26]. In addition, considering the restrictions on water resources anticipated in the coming years due to climate change [
93,
94,
95], measures should be considered to increase rainfed agricultural yields, especially in Mediterranean areas characterised by water stress.