Parameter Adjustment to Prevent the Overestimation of Peak Discharge in a Small Watershed

We revisit empirical methods to prevent the overestimation of peak discharge in a small watershed, in particular investigating the time-area method, which has not been considered in the overestimation problem of peak discharge. To avoid misapplying the same inlet time between the unit hydrograph and rational formula, distinct parameter adjustments for each method are proposed. We adopt the secondary basin response time for the unit hydrograph, rainfall duration for the rational formula, and time of concentration for the time-area method, as suitable parameters to adjust the estimation of peak discharge. In conclusion, adding 10 minutes to secondary basin response time, 20 minutes to rainfall duration, and 30 minutes to time of concentration, respectively, yields estimates within a reasonable range of specific discharge in a small watershed.

Accuracy Assessment of Post-processing Kinematic Georeferencing Based on Real-time Kinematic Unmanned Aerial Vehicle and Structure-from-motion Photogrammetry: Topographic Measurements of a Riverbed in a Small Watershed with a Check Dam

<p><a>To obtain an accurate digital surface model of the small watershed topography of a forested area while reducing time and labor costs, we used a consumer-grade unmanned aerial vehicle (UAV) with a build-in real-time kinematic global navigation satellite system. The applicability of structure-from-motion (SfM) multi-view stereo processing with post-processing kinematic (PPK) correction of the positional coordinate data (the UAV-PPK-SfM method) was tested. Nine verification points were set up in a small (0.5 km²) watershed, based on a check dam in the headwaters of a forest area. The location information of the verification points extracted from the digital surface model acquired by UAV-PPK-SfM and the overall working time were compared with the corresponding location information and working time of a traditional field survey using a total station. The results showed that the vertical error between the total station and each verification point at an altitude of 150 m ranged from 0.006 to 0.181 m. The working time of the UAV-PK-SfM survey was 10 % of that of the total station survey (30 min). The UAV-PPK-SfM workflow proposed in this study shows that wide-area, non-destructive topographic surveying, including fluvial geomorphological mapping, is possible with a vertical error of ±0.2 m in small watersheds (<0.5 km²). This method will be useful for rapid topographic surveying in inaccessible areas during disasters, such as monitoring debris flow at check dam sites, and for efficient topographic mapping of steep valleys in forested areas where the positioning of ground control points is a laborious task.</a></p>

Accuracy Assessment of Post-processing Kinematic Georeferencing Based on Real-time Kinematic Unmanned Aerial Vehicle and Structure-from-motion Photogrammetry: Topographic Measurements of a Riverbed in a Small Watershed with a Check Dam

<p><a>To obtain an accurate digital surface model of the small watershed topography of a forested area while reducing time and labor costs, we used a consumer-grade unmanned aerial vehicle (UAV) with a build-in real-time kinematic global navigation satellite system. The applicability of structure-from-motion (SfM) multi-view stereo processing with post-processing kinematic (PPK) correction of the positional coordinate data (the UAV-PPK-SfM method) was tested. Nine verification points were set up in a small (0.5 km²) watershed, based on a check dam in the headwaters of a forest area. The location information of the verification points extracted from the digital surface model acquired by UAV-PPK-SfM and the overall working time were compared with the corresponding location information and working time of a traditional field survey using a total station. The results showed that the vertical error between the total station and each verification point at an altitude of 150 m ranged from 0.006 to 0.181 m. The working time of the UAV-PK-SfM survey was 10 % of that of the total station survey (30 min). The UAV-PPK-SfM workflow proposed in this study shows that wide-area, non-destructive topographic surveying, including fluvial geomorphological mapping, is possible with a vertical error of ±0.2 m in small watersheds (<0.5 km²). This method will be useful for rapid topographic surveying in inaccessible areas during disasters, such as monitoring debris flow at check dam sites, and for efficient topographic mapping of steep valleys in forested areas where the positioning of ground control points is a laborious task.</a></p>

Characteristics of Soil Nitrogen and Phosphorus in Different Types of Areas in the Pisha Sandstone Region

Little is known about the distribution of N and P in Pisha sandstone soils from different types of areas. In this study, three typical areas in this region, which included the Er Laohu small watershed (soil-covered area), the Tela small watershed (sand-covered area), and the Shibu Ertai small watershed (bare area), were chosen as experimental objects. A total of 27 soil sections were constructed (Took three parts in the upper, middle and lower of each watershed, respectively), and 486 soil samples were taken from different slope positions and slope directions. Bare Pisha sandstone had the highest mean TN concentration of 0.53 g/kg, which was about 45.06–45.30% higher than soil-covered and sand-covered Pisha sandstone. Sand-covered Pisha sandstone had a lower TP concentration of 37.93–40.62% than the bare (5.04 g/kg) and soil-covered (4.64 g/kg) Pisha sandstone. The concentration of AN in the bare Pisha sandstone was 23.98 mg/kg, which was about 2.85-3.85 times higher than that in the sand-covered and soil-covered Pisha sandstone. Ni concentration in bare Pisha sandstone (5.14 mg/kg) was approximately 3–4 times higher compared to soil-covered and sand-covered Pisha sandstone. The AP concentration in soil-covered Pisha sandstone was 7.56 mg/kg, which was about 68–72.21% higher than that in bare and sand-covered Pisha sandstone areas. Overarchingly, our results suggest that different Pisha sandstone types and slope positions significantly affect the spatial distribution of N and P. That has created a greater obstacle for the local population to carry out agricultural activities in the Pisha sandstone region.