## 4 Soil and Water Conservation and Its Impacts on Hydrology and Soil Erosion
### Impact on Runoff
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 _Khuya 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]. \begin{table}
\begin{tabular}{c l c c c} \hline \hline \multirow{2}{*}{**No.**} & \multirow{2}{*}{**Cover Crop; Hydrology Condition**} & \multicolumn{3}{c}{**Runoff Coefficient for Rainfall Rate**} \\ \cline{3-5}  & & **25 mm/h** & **100 mm/h** & **200 mm/h** \\ \hline
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, & 0.29 & 0.36 & 0.39 \\  & crop rotation; good & & & \\
6. & Grass, & 0.02 & 0.17 & 0.23 \\
7. & Forest; good & 0.02 & 0.10 & 0.15 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Successful indicator for soil and water conservation according to the tolerable runoff coefficient. 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.