Check dam identification using multisource data and their effects on streamflow and sediment load in a Chinese Loess Plateau catchment

Abstract During the recent six decades, numerous check dams have been constructed for soil erosion control and agricultural production, and have become the key measure for soil and water conservation in the Loess Plateau. Obtaining check dam information is very important for soil erosion control and river basin management. This study utilizes remote-sensing images in conjunction with Google Earth images and field survey to derive the spatial distribution of the check dams in the Huangfuchuan catchment. Multisource data including topographic maps, Landsat images, and images from Google Earth are collected for check dam identification. The results are verified based on a field survey. The water surface area of the check dams derives from in situ measurement and images present good relationship with the high correlation coefficient of 0.96. Furthermore, the area extent and number of check dams derived from the remote sensing images are similar to those from Google Earth images. Historically, an increase in the check dam number, controlled area, and storage capacity indicate their substantial trapping effects on streamflow and sediment load in the Huangfuchuan catchment. This study may be a good reference for proposing an efficient approach to identify the check dams and provide decision supports for soil and water conservation in the Loess Plateau.

sediment yield through both field measurement and numerical simulation. It has been concluded that the check dams captured sediments and led to a decrease in the longitudinal gradient upstream, but accelerated downstream erosion because of the increased flow transport capacity and scour processes. 1,4,5 Bombino et al. 6 addressed statistically significant differences in channel form, sediment, and vegetation among the upper, the lower reaches, and the intermediate sections of check dams in Southern Italian catchment. A reducing rate of 77% sediment yield by check dams has been reported by Boix-Fayos et al. 2 in the Rogativa catchment of SE Spain. Ran et al. 7 analyzed the coarse sediment retention by check dams in five typical catchments in the midstream of the Yellow River, and found that the average sediment reduction ratio could reach 60% when the check dams area accounted for more than 3% of the drainage area.
In Chinese Loess Plateau, check dams have attractive advantages because of unique environmental settings and regional food supply needs. 5,8,9 For >400 years, check dams have been constructed for soil erosion control and agricultural production improvement. The check dams have become one of the most effective strategies for soil erosion control in the Loess Plateau. Especially during the recent six decades, thousands of check dams have been constructed in the region. By 2005, >120;000 check dams have been built on the Loess Plateau over the past 50 years and approximately 21 billion m 3 of sediments have been captured by these dams. 10 Although considerable studies have been conducted as mentioned above, the locations, storage capacities, and outflow methods have not been inventoried and documented. Thus, further understanding the mechanisms on the hydrological effects of check dams was impossible.
Traditionally, check dams and their storage capacity are estimated by using the topographic maps according to the in situ hydrographic survey with an electronic planimeter. 11 However, this approach is usually laborious, costly, and time consuming. Advances in remote sensing, geographic information systems, and computer technology have made satellite images convenient and less expensive for reservoir or dam measurement and monitoring. 12,13 Remote-sensing images provide invaluable and timely information on spatial distribution, spectral and temporal attributes for reservoirs, and dam operation and management. 14,15 However, research on check dam identification and quantitative estimation of storage capacities by means of remote-sensing images are very rare. 16,17 The few studies available mainly focused on individual reservoirs or a rather small watershed and as a result, only some tentative applications have been reported. 18 To better understand the potential effects of check dams on hydrological processes and sediment yield at catchment scale, an accurate estimation of the check dam distribution and their storage volume becomes very important and necessary. The objectives of this study, therefore, were: 1. to identify the check dams in a Loess Plateau catchment by making use of multisource data; 2. to quantify the actual storage capacity of the check dams through established empirical relationships; and 3. to analyze the impacts of check dams on streamflow and sediment load in combination with recently observed dataset.
2 Study Area and Data

Study Area
The study area, Huangfuchuan catchment, is one of the most severe soil erosion areas with the average annual sediment modulus higher than 17; 000 ton∕km 2 (1955 to 2000). The Huangfuchuan River is a first-order tributary of the midstream Yellow River, located in the wind-water erosion criss-cross region. The river originates from the eastern part of the Ordos Plateau, flows through the transitional zone of the Loess Plateau and Inner Mongolia grassland, and empties into the main stream of the Yellow River in Fugu County of Shaanxi Province (Fig. 1). The Huangfuchuan catchment covers an area of 3246 km 2 consisting of two subtributaries: Nalin River and Changchuan River. The main stream of the Huangfuchuan River is 137 km long with an average channel slope of 2.7%. The watershed is located in the transitional belt of warm temperate and mesothermal zones with the average annual precipitation of about 380 mm, influenced by southeast monsoon in summer, and characterized by semi-arid continental climate. 19 Frequently occurring floods in July and August often cause tremendous soil erosion in the catchment. Average annual discharge and sediment load are 1.269 × 10 8 m 3 and 0.408 × 10 8 ton, respectively, between 1955 and 2010, nearly 80% of which is concentrated in the rainy season from June to September. The catchment can be described as "hilly and gully" landscape, and the land surface material is dominated by weathering sandstone, silty loess, and desert sand with coarser particles. According to the soil erosion type, the basin can be divided into three subregions: (1) loess hilly-gully region with an area of 918.3 km 2 , mainly located in the eastern and southwestern parts of the watershed; (2) the sandy loess hilly-gully region with an area of 546.1 km 2 , located between Nalin and Changchuan tributaries; and (3) the sandstone weathering hilly-gully region with an area of 1781.6 km 2 , located in the northwestern part of the catchment. Due to the extremely high gully density and soil erosion rates, check dams have become the dominant soil and water conservation measures in the Huangfuchuan catchment. By 2010, approximately 70% of the area in the Huangfuchuan watershed was controlled by check dams.

Available Data
The data used in this study were collected from topographic maps (1978), Landsat images (1990,2007), and images acquired in 2010 from Google Earth. Table 1 lists all the available datasets and sources. In total, we collected 28 topographic maps with the resolution of 1:50,000 from Bureau of Survey and Mapping Shaanxi Province. The images were downloaded from the Landsat imagery database. 20 The coverage of an individual Landsat TM image is 185 × 185 km 2 . The resolution for bands 1 to 5 and 7 is 30 m, while it is 120 m for band  3 Methodology

Check Dam Identification from Multisource Data
The topographic maps were geocoded to universal transverse mercator (UTM), zone 49N, using the WGS84 ellipsoid, and were used to delineate the check dams in ArcMap manually. Landsat images were subjected to atmospheric correction, radiometric calibration, and radiometric rectification procedures to facilitate comparability between dates. 22 The Landsat images were georeferenced to 1:50,000 scale topographic maps using second-order polynomial transformation with the root mean square error of below 0.5 pixels (<15 m), and registration was verified visually by overlaying and swiping registered images. Due to the strong absorption and reflectance region in the infrared, visible red, and near-infrared bands (bands 7, 4, and 3) provided in the Landsat images, the combination of these bands was used to distinguish water bodies and land.
The check dams or reservoirs were extracted by using the maximum likelihood supervised classification method in Erdas Imagine software. 14 Currently, Google Earth has provided images of resolution of <15 m covering the entire river basin. 21 In addition, GPS field survey has similar accuracy of 10 to 20 m for ground truthing. 11,15 Thus, we derived the spatial distribution and area extent of the check dams to verify the interpretation results of Landsat images. The images displayed by Google Earth were taken on June 23, 2010, coinciding with the capture time of the Landsat images. An update dataset for check dams in the Huangfuchuan catchment was obtained through images interpretation from Google Earth Pro.
Field survey was carried out in the beginning and end of the rainy season (May 2012 and October 2012). The outlines of the check dams were obtained with a Garmin 76 GPS instrument. The data stored on the GPS was converted into text formats which can be read by ArcCatalog. The points were connected together to form polygons. The surface areas and perimeters were then calculated.
To assess accuracy of the extracted check dam area from different data sources, both normalized difference area index (NDAI) and deviation area index (DAI) were applied. 14,23 NDAI and DAI are defined as follows: The NDAI value falls within the range between −1 and 1 with values close to 0 giving the best fit between the surface areas obtained from GPS and other sources, respectively. Values increasing to both extremes indicate increasing deviation between Area gps and Area img . Similarly, the DAI values also lie between −1 and 1, with values close to 0 meaning the best linear fit, whereas values increasing to both extremes indicate larger deviation between Area gps and Area img .

Storage Capacity Estimation
If reservoirs or check dams can be assumed to approximate the shape of a square pyramid diagonally cut in half, 15,21,24 an empirical relationship between the storage capacity and its area can be expressed as where V is the check dams' storage capacity, Area is its water surface area, and a and b are the constants to be determined. If reservoirs are indeed exactly half pyramids, then b ¼ 1.5. When the slopes are more convex (rare), the constant b is >1.5. In most cases of concave slopes, the value of b is <1.5. Particularly, the parameters a and b are surprisingly constant within geomorphologically similar regions. 15 However, it should be noted that the parameters may differ greatly in regions with geomorphologic heterogeneities. 14,21,23 Thus, more attention should be paid when using the empirical relationship. To further verify the water surface area derived from Landsat images, field surveys in 11 and 14 check dams were undertaken in May and October 2012, respectively. The NDAI and DAI were applied to detect the interpretation results. Table 2 compared the water surface area in check dams derived from images and field measurements. There is a very good linear fit between these two datasets. In most cases, we found that the water surface area from the Landsat images has overestimated the measurement values. Probably, the overestimation may result from the lower resolution of the Landsat images by which the surrounding nonwater features would have been misinterpreted as water. Furthermore, the results of supervised classification are largely dependent on the accuracy of the training data and their representative. In addition, the differences in water surface area may generate great discrepancies because of the uneven distributed

Storage Capacity Estimation in the Catchment
In total, datasets of 74 check dams with spatial distribution, built date, storage capacities, and dike height were provided by the Yellow River Basin Committee. By representing various check dams with different storage capacities, these check dams were employed to develop the relationship between the water surface area and the storage capacity. As shown in Fig. 4, we investigated the relationship between the storage capacity and the water surface area as well as the damcontrolled area. The results indicate relatively good power relationship between the water surface area and the storage capacity, and logarithmic relationship between check dam controlled area and storage capacity. In contrast, Fig. 4(a) confirmed that the power equation represents a better relationship between check dam water surface area and storage capacity [ Fig. 4(b)]. Thus, an empirical equation for calculating the check dams' storage is Equation (4) was applied to estimate the storage capacity of each check dam, and the total storage capacity was estimated by summing all individual check dams' storage together ( Table 3). The total storage capacities in the Huangfuchuan catchment have increased from 2.65 × 10 8 m 3 (1978) to 4.11 × 10 8 m 3 (2010), and the average storage capacity for the check dams is nearly 7.25 × 10 5 m 3 . The estimated errors for storage capacities mainly result from the spatial heterogeneities in geometric shape and extracted water surface area. Ran and Lu 21 addressed that each different geometric shape requires a unique parameterization to precisely calculate its storage capacity. In addition, the over-or underestimated water surface area may lead to misestimation of storage capacity for the check dams. However, compared with traditional field survey approach, the proposed method in this study was useful, efficient, and less expensive, and the estimation accuracy is satisfactory.    2 , accounting for about one third of the whole catchment. By 2010, the number of check dams had reached 567, and the dam-controlled area was 2216.47 km 2 . This is consistent with the field survey data reported by the Hydrology Bureau of the Yellow River Water Resources Commission. Minor discrepancies were detected between Jiao et al. 25 and our estimation. They addressed that 392 major check dams had been constructed until 1992 in the Huangfuchuan catchment, and approximately 1.07 × 10 8 m 3 sediment had been trapped behind the check dams. . Considerable evidence exists that check dams' construction was responsible for the significant reduction in both streamflow and sediment load. 8,10 Ran 28 addressed that streamflow reduced by soil and water conservation measures ranged from 0.14 to 0.53 × 10 8 m 3 from 1970 to 1996. Average annual reducing rates for sediment load lay in the range of 5.4 to 23.6 Mt. Among these, check dams are the dominant factors retaining discharge and sediment in the watershed. 29 Furthermore, sediment trapped by the check dams accounted for >60% of the total reduced sediment load in the Huangfuchuan catchment.

Potential Impacts of Check Dams on Streamflow and Sediment Load
To better understand the impacts of check dams on streamflow and sediment load, the hydrological time series at Huangfu station were divided into two periods (1955 to 1979 and 1980 to 2010) by the Mann-Kendall test method. A referenced period from 1955 to 1979 was assumed to be the period without intensive human influences on the hydrological process and the changing period was between 1980 and 2010, which was highly affected by human activities. Figures 6(c) and 6(f) show the daily flow duration curves within two different periods. The magnitude of the daily flow generally decreased during the changing period compared with the referenced period.    It can also be clearly seen that zero discharge accounts for much more during the changing period. The daily sediment delivery ratio with 0 values is nearly 60% within 1955 to 1979 but reduced to 32.6% during the changing period. There is no discharge measured at Huangfu station during all of 2011. Even a storm in 14th of August with 58.4-mm rainfall did not produce surface flow. 30 Thus, it can be concluded that numerous soil and water conservation measures, especially, the check dams trapped considerable discharge and sediment in the Huangfuchuan catchment.

Sediment Particle Sizes Associated with Check Dam Construction
In general, hyper-concentrated sediment flow can be trapped in the check dams. In most cases, the check dams consisted of the embankment, spillway, and outlet. Before 1980, most check dams had no spillway or outlet, and all the upstream incoming sediment would be trapped by the dams. In the check dams with spillway, the sediment flow will be filtered. In principle, the coarse grain sediment will deposit rapidly in the water surface area of the dam, and the sediment with fine particle sizes will be flushed away through the spillway. In such cases, most coarse-grain sediment is trapped before the dams.
In the Huangfuchuan catchment, frequently occurring storms mainly concentrated in July and August. Sediment load during these months contributed approximately 70% of the total. In recent years, zero flow and sediment lasted several months during the dry season. Thus, sediment particle sizes during July and August may represent the effects of check dams on them. As shown in Table 4, observed data at Huangfu hydrological station indicated that mean and median sediment particle diameter illustrated evident decreasing trends, presenting the trapping effects of the check dams. The percentage of sediment particle diameter <0.025 mm increased evidently from 1966 to 2010. Both the median and mean value of sediment particle diameter displayed particularly low values during 2005 and 2010 (bold values in Table 4). This is similar to the diameter changes in the middle reaches of the Yellow River. 28,29

Driving Factors of Check Dam Construction
In most cases, the check dams were constructed for soil erosion control. 31 However, other objectives such as improving agricultural productivities and building railway or highway become dominant factors. As shown in Fig. 7(a), nearly 10 check dams were constructed for protecting the highway in a 5-km 2 region of the Huangfuchuan catchment. Road construction has become an important forcing variable for check dam building. Statistics indicate that nearly 20% of the check dams are connected with the road or used as river bridges. On the other hand, the major aim for building the check dams is to increase agricultural production [ Fig. 7(b)]. In general, food production in dam cropland can be 6 to 10 times higher than that in the slope cropland. The average yield is 45; 000 kg ha −1 with some even up to 105; 000 kg ha −1 . 32 Accordingly, dam cropland accounts for about 9% of the total cropland area in the Loess Plateau, whereas the food production amounts to 20.5% of the total food production. 8

Conclusion
This study identified the spatial distribution, water surface area, and controlled area of the check dams by making use of topographic maps and remote-sensing images in combination with a field survey. An empirical equation was applied to estimate the storage capacity of the check dams associated with the water surface area. Potential effects of the check dams on streamflow and sediment load were additionally addressed. The conclusion of the study can be summarized as follows: A good correlation between the measured and derived water surface area of the check dams from maps and images was detected. The check dams' information extracted from the remote sensing images is the same as those from Google Earth images, suggesting that the topographic maps and Landsat images can be used for identifying the check dams. Historically, the number of check dams has increased from 390 in 1978 to 567 in 2010, and the dam-controlled area has reached up to 2216.47 km 2 , accounting for nearly two thirds of the whole catchment. The large numbers of check dams indicate their substantial trapping effects on streamflow and sediment load in the watershed. Filtering effects of check dams have been confirmed due to evident decreasing sediment particle sizes. This study proved that the proposed approach can be applied for extracting check dam information and may provide decision supports for soil and water conservation in the Loess Plateau.