Encyclopedia
Please note this is an old version of this entry, which may differ significantly from the current revision.
Subjects:
Environmental Sciences
Soil and water are natural resources that support the life of various creatures on Earth, including humans. The main problem, so far, is that both resources can be easily damaged or degraded by human-induced drivers. The threat of damage or degradation is increasing due to rapid human population growth and humans’ insatiable daily necessities.
- biodiversity
- hydrology
- landscape ecology
- microclimate
- soil and water conservation
1. Soil and Water Conservation and Its Impacts on Biodiversity
Indonesia is one of the seventeen global megadiverse countries, with two out of the twenty-five biodiversity hotspots in the world [40]. However, the rate of primary forest loss in Indonesia is double that of the Democratic Republic of the Congo and triple that of Brazil [41]. The drivers of forest loss in Indonesia range vastly from the industrial-scale logging of montane rainforests to the small-scale destruction of coastal peatlands. The loss of forest cover that human-induced factors have caused impacts Indonesia’s biodiversity at various levels. Although the type of forest loss varies widely across the landscape, the continued exploitation of primary tropical rainforests remains the most prevalent type of forest loss in Indonesia [41].
In order to address long-term concerns in biodiversity conservation, Indonesia must strike a balance between protection and sustainable utilization to its national biodiversity. Thus, SWC becomes a crucial component of the environmental sustainability of forestry systems [42]. SWC’s favourable impact on vegetation cover, plant diversity, richness, and restoration is logical [43]. On the other hand, vegetation has been an essential component in the watershed ecosystem, providing a buffer element for soil erosion and drought [44] and increasing soil’s capacity to hold water [45]. The diversity of tree species in upstream watersheds, such as Alstonia scholaris, Arenga pinnata, Artocarpus heterophyllus, and the commonly dominant species Pinus merkusii, has essential roles in maintaining soil conservation and forest ecosystems in general [46]. Likewise, Coffea arabica, an important crop and vital product in several highland countries, contributes significantly to water conservation [47]. Other species that can be cultivated for vegetative SWC are Tectona grandis, Delonix regia, Switenia mahagoni, Cassia siamea, and Samanea saman [48]. Tree-level vegetation has various characteristics that can affect surrounding environmental conditions.
Additionally, the establishment of understorey plants that have the potential to maintain SWC in the upstream area is urgently needed. A previous study proved that Mimosa pudica, Ageratum conyzoides, and various other understorey plants possess a high potential for conserving soil in watershed areas [44]. The functions of ecosystems’ soil conservation can be improved through vegetation restoration, which can also successfully lessen soil erosion [49]. Grassy vegetation covers can hold the surface runoff and increase water infiltration into the soil [50]. The presence of vegetation will positively impact the ecosystem’s balance on a broader scale in a complex landscape system (Figure 1) [51].
Figure 1. Key components related to SWC in the ecosystem management.
In other words, the presence of forest vegetation and its litter layers may reduce the amount of surface runoff and convert rainwater into groundwater supply [52]. Such a condition is vital in maintaining soil stability and reducing soil loss in steep slopes [53]. Knowing that each vegetation type has its respective role and understanding the potential of runoff suppression plants and their role in supporting SWC are crucial. As important resources, soil and water can be preserved by utilizing native plants adapted to local rainfall and soil conditions. Local communities believe that native plant species have the ability to maintain the existence of natural springs and preserve the soil. To ensure better management and utilization of natural resources, local communities are expected to be able to collaborate with local and national governments [54].
2. Soil and Water Conservation and Its Impacts on Hydrology and Soil Erosion
Massive development and population growth have caused land use changes that affect the hydrological response of a watershed [55]. In a watershed management plan, reduced hydrological function due to land degradation is the main target for rehabilitation [56]. A degraded watershed is characterized by minimal base flow during the dry season, high direct runoff during the rainy season, erosion, and sedimentation exceeding the threshold. Continuous erosion will excessively shed essential nutrients and organic matter on the soil surface and can inhibit the soil’s physical, biological, and chemical functions. This condition will also reduce the hydrological properties of the soil due to minimal soil organic matter content and is accompanied by a decrease in aggregate stability and water-holding capacity [57].
2.1. Impact on Runoff
In Indonesia, most watershed degradation has been caused by changes in land cover. It is accompanied by an increase in runoff, especially in the upstream region, initially in the form of forest cover [58]. Deforestation and forest degradation can affect forest hydrological function. This function is essential for managing water, regulating the microclimate, and mitigating disaster [59].
Afforestation on a wide scale can suppress erosion and runoff, but attention must be paid to determining the composition and species used, especially in dry climates. The selected species have ecological consequences. Therefore, it is important to consider the climate conditions, soil hydrological characteristics, and landscape forms. Improper species determination results in water shortages and exacerbates land degradation [60]. Previous research on soil’s hydrological properties reported differences in the effects of three tree species [59]. Pinus merkusii had a more significant effect on fluctuations in soil water content (3.1%) compared to Hopea odorata (2.4%) and Khaya anthotheca (1.9%). For the infiltration variable, P. merkusii, H. odorata and K. anthotheca had values of 95.5, 27.9, and 28.6 cm/hour, respectively, while their permeability values were 9.2, 8.1, and 8.7, respectively.
Improvement in the hydrological function of a watershed through SWC can be carried out on the principle of reducing slope and slope length. Constructive or vegetative measures can also increase infiltration and improve soil hydrological characteristics [61,62]. Improvement in the hydrological quality of a watershed is characterized by a decrease in the flow regime coefficient (FRC), i.e., the ratio of maximum discharge to minimum discharge; a reduction in the annual flow coefficient (AFC), i.e., the balance between runoff and rainfall amount in a year; a reduction in direct runoff; and increases in base flow and water yield [63]. At Ciliman Watershed (Banten Province), the application of SWC reduced FRC, AFC, and direct runoff by 31.6%, 24%, and 23.6%, respectively, and increased base flow and water yield by 16.2% and 1.8%, respectively [64]. The application of SWC at upper Cimanuk Watershed (West Java) reduced the flow regime coefficient, reduced runoff by 40.8%, and increased lateral flow by 536.9 mm [65]. At the upper Opak Watershed, the application of SWC was able to reduce runoff by 27.1%, increase base flow by 18.3%, and improve FRC and AFC values [56]; meanwhile, at the upper Cisadane Watershed, SWC reduced runoff [58].
The choice of an SWC technique should be adjusted to local characteristics such as the amount of rainfall, slope, soil type, and crop species. The difference in these characteristics affects the effectiveness in suppressing runoff, which averages 13–71%. In 2014, the National Standardization Agency of Indonesia released the Indonesian National Standard (SNI) 7943:2014 about soil and water conservation guidance for addressing degraded land. The SNI document stated that successful indicators for soil and water conservation are tolerable erosion value and runoff coefficient according to the soil type, land use, and management. Runoff coefficients are presented in Table 1 [66].
Table 1. Successful indicator for soil and water conservation according to the tolerable runoff coefficient.
| No. | Cover Crop; Hydrology Condition | Runoff Coefficient for Rainfall Rate | ||
|---|---|---|---|---|
| 25 mm/h | 100 mm/h | 200 mm/h | ||
| 1. | Plants in rows; bad | 0.63 | 0.65 | 0.66 |
| 2. | Plants in rows; good | 0.47 | 0.56 | 0.62 |
| 3. | Paddy; bad | 0.38 | 0.38 | 0.38 |
| 4. | Paddy; good | 0.18 | 0.21 | 0.22 |
| 5. | Grass, crop rotation; good |
0.29 | 0.36 | 0.39 |
| 6. | Grass, steady development; good |
0.02 | 0.17 | 0.23 |
| 7. | Forest; good | 0.02 | 0.10 | 0.15 |
Dunes accompanied by silt pits, bench terraces, tied ridges, and mulch can reduce runoff by 51–57%, while rock mounds can suppress it by 50–86%. By applying grass strips and hedgerows, runoff can be reduced by 56 and 61%, respectively, even up to 77%, depending on the plant species and density [57].
In oil palm plantations, the application of bunds can reduce surface runoff by 63.4%. Its effectiveness is proven to be higher through a combination with the Nephrolepis biserrata cover crop, with a runoff reduction of 95.7% [67]. Cover crops can also be effectively combined with sediment traps and manures to reduce runoff by 127.8 m3/ha [68].
Organic materials derived from plant parts and animal waste, known as mulch, are potent in SWC. Apart from being a soil nutrient enhancer, it is also related to the presence of and increase in soil macrofauna activities which can improve soil structure, stability, and porosity and accelerate decomposition [69]. The presence of mulch and soil macrofauna significantly reduces runoff and increases soil moisture [70]. Mulch has also been effective in suppressing evaporation and creating a more comfortable growing place for plants [71,72]. The SWC technique is applied during reforestation using Khaya anthotheca. It involves vertical mulching at 6 m intervals, which effectively reduces runoff by 75%, or 12 m intervals, which reduces runoff by 41% [31]. The application of mulch combined with ridge furrow has been shown to increase water use efficiency and crop yields [73], including in areas that often lack water [74]. The combination of the two SWC techniques is proven to increase soil water content to a depth of 50 cm, maintain soil water storage to a depth of 110 cm, and increase the growth and production of crops [72].
Soil “bunds” effectiveness is influenced by the distance between bunds. The narrow spacing between bunds effectively accommodates surface runoff in the bund channels and increases the infiltration rate of surface runoff water. Observations for two years on a slope of 9%, with a distance between ridges of 5 m, found a reduction in runoff to 53%, 2.2 times lower than controls [61].
SWC’s ability to suppress runoff is also followed by increased soil moisture. The combination of contour ridges and infiltration pits can maintain soil moisture within a radius of about 3 m, especially on gentle slopes, with range decreasing as the slope increases [75].
The application of soil bunds can also increase soil moisture (soil water content). Soil water content increases as the distance between the ridges (i.e., the slope distance) decreases. It is due to the increasing infiltration in the soil medium between bunds since mounds can reduce the speed of water flow and delay runoff. To find the optimal spacing of soil bunds in each region requires exploring the effect of bund spacing on different landscape biophysical aspects, economic feasibility, and social acceptability [61].
Rainfall intensity and slope conditions influence the amount of surface runoff. Observations on the watershed scale show that SWC in contour ridges and hillside reservoirs that is applied to 21% of the watershed area can reduce runoff by 41–50%, especially if the rainfall intensity is 40 mm/event [76]. On average, contour ridges that are applied to 43% of the catchment area can suppress 50–80% of runoff. If the rainfall event is below 70 mm, the SWC used can reduce runoff by up to 95%, and if the rainfall reaches 80 mm, the runoff can be reduced by 75% [77].
On tea plantations with slopes of 8–35%, a contour trenches treatment, combined with cover crops, reduces runoff to half compared to plantations that do not apply SWC. This SWC technique also increases soil moisture from 18.6% to 25.1%. Higher soil water content can be associated with the retention of runoff in trenches, followed by absorption in the soil profile in the root zone [61]. In tropical date palm plantations, silt pits reduce surface runoff by 88.55% [78].
The occurrence of erosion changes soil characteristics. The soil becomes difficult for plants to grow, and infiltrated rainwater becomes limited compared to runoff. This condition makes water availability for plants increasingly limited, threatening food security and the environment [69,79]. Efforts to increase water availability for plants through water infiltration are carried out by suppressing evaporation and runoff. The selection of SWC techniques, water harvesting techniques (WHT), and water use efficiency can be adjusted according to local conditions. Excessive infiltration triggers landslides in areas prone to landslides; thus, surface runoff must flow through drainage channels to storage areas/reservoirs.
The use of water harvesting techniques is not only restricted to supplying water for agriculture but also reduces the amount and velocity of runoff, which causes erosion of the fertile topsoil [80]. At Cilemer Watershed (Banten Province), constructing a reservoir as an alternative to SWC reduced direct runoff by 29.2%, decreased AFC from 0.25 to 0.17, increased base flow by 46.0%, and increased water yield by 3.99% [63]. At Pesanggrahan Watershed, creating six reservoirs reduced 24.6% of peak flow, while the current conditions are only able to reduce 6.4% [81].
Applying a hedgerow barrier has the main purpose of SWC. However, hedgerow plant biomass as an auxiliary product can be used directly as food, animal feed, and firewood. Soil equipped with a hedgerow barrier can increase infiltration up to eight times compared to the surrounding area due to improved macro soil porosity from increased organic matter and root canals [82].
2.2. Impact on Soil Erosion
Indonesia’s topography is dominated by sloped land, and most areas have high rainfall. With a high population with a majority of farmers, making agricultural land with ideal conditions is increasingly difficult to obtain. The expansion of agricultural land and plantation is mostly carried out in sloping areas, which are often not accompanied by SWC measures, which contributes greatly to soil erosion [9,10]. If this condition is not handled, it will lead to a decrease in land productivity and can result in more severe disasters. Appropriate SWC application not only suppresses runoff but effectively reduces erosion. One of the successful indicators for soil and water conservation can be indicated by tolerable erosion value. Tolerable erosion values, as determined by the Indonesian National Standard (SNI) 7943:2014 are presented in Table 2 [66].
Table 2. Successful indicator for soil and water conservation according to the tolerable erosion value.
| No. | Soil Characteristics and Substratum | Tolerable Erosion Value (mm/year) |
|---|---|---|
| 1. | Very shallow soil over rocks | 0.0 |
| 2. | Very shallow soil over weathered material (unconsolidated) | 0.4 |
| 3. | The shallow soil above the material has weathered | 0.8 |
| 4. | Soil of medium depth above the material has weathered | 1.2 |
| 5. | Deep soil with impermeable subsoil water on a weathered substrate | 1.4 |
| 6. | Deep soil with undersoil slow permeability, on top of the substrate has weathered | 1.6 |
| 7. | Deep soil with subsoil medium permeability, above the substrate has weathered | 2.0 |
| 8. | Deep soil with a deep permeable subsoil, over the substrate has weathered | 2.5 |
Remarks: Soil of deep depth (>90 cm); soil of medium depth (50–90 cm); soil of shallow depth (25–50 cm); soil of very shallow depth (<25 cm).
In oil palm plantations, several SWC methods have been tried, such as bench terrace, individual terrace, cover crop, sediment trap, vertical mulch, and some combinations of measures. The measurement results using erosion plots show that SWC measures are more effective if they consist of two or three measure combinations. The combination of bench terrace, cover crop, and manure produces a reduction in the erosion of up to 70% compared to the control, while the other best combination is a combination of silt pit, cover crop, and manure with a reduction of up to 60% [68]. The application of silt pits has also proven to be effective in suppressing erosion in date palm plantations at Aceh Province. By implementing silt pits, erosion that occurs during extreme rainfall can be reduced by up to seven times [78].
The application of SWC is not only effective in agricultural and plantation lands, but has also proven effective when applied to reforestation areas. The use of vertical mulching can reduce erosion by 37% at 6-m intervals, while at 12 m intervals, erosion can be reduced by 30%. A significant reduction in erosion is also accompanied by five times the prevention of nutrient loss compared to the control [31].
This entry is adapted from the peer-reviewed paper 10.3390/app13137648
This entry is offline, you can click here to edit this entry!
