## 4 Soil and Water Conservation and Its Impacts on Hydrology and Soil Erosion
### 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 _Khuya 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 2[66]. 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 m\({}^{3}\)/ha [68]. Organic materials derived from plant parts and animal waste, known as mulch, are potent in SWC.