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Sampling Accuracy in Erosion Plot Tanks

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Sampling Accuracy in Erosion Plot Tanks Proceedings of The Fourth International Iran & Russia Conference 1245 Sampling Accuracy in Erosion Plot Tanks Davood Nikkami 1 , Mahmood Arabkhrdri 2 , and Peyman Razmjoo 3 1 Assistant Professor, Soil Conservation and Watershed Management Research Institute, P.O. Box: 13445-1136, Tehran, Iran, Tel: (9821) 490-5875, Fax: (9821) 490-5709, e-mail: [email protected] 2 Research Assistant, Soil Conservation and Watershed Management Research Institute (SCWMRI), P.O. Box: 13445-1136, Tehran, Iran, Tel: (9821) 490-1214, e-mail: [email protected] 3 Research Assistant, Soil Conservation and Watershed Management Research Institute (SCWMRI), P.O. Box: 13445-1136, Tehran, Iran, Tel: (9821) 490-1214, e-mail: [email protected] Abstract Effective factors in soil erosion process and sediment yield are usually evaluated in soil erosion and sediment study plots. Investigating and evaluating effective factors are based on runoff and sediment samples from tanks which are located at the end of these plots. Although these plots and their accouterments are constructed precisely in right places, significant errors on samplings lead the researchers to make wrong decisions. There are limited researches on sampling accuracy from these tanks. In this article that is the result of a research implemented in the Soil Conservation and Watershed Management Research Institute (SCWMRI) during 2003, the sampling accuracy of bottle, pipette, and cylindrical sampler was investigated. Three concentrations of 1.87, 4.68 and 9.36 gr/l of sediment were prepared in 220 liter plot tanks with three mixing periods of 1, 2 and 5 minutes. The samples were taken from center and side of the tanks and from the depths of 20, 40, 60 and 80 centimeter from the water surface for bottles and pipette and the whole depth for cylindrical sampler. The cylindrical sampler showed the least error of 13.04% on concentration compared to two other methods. Sampling with bottle and pipette illustrated that the concentration of sediment increases and becomes more accurate with the depth of the tank. The results showed the most accurate concentration in the last 20 centimeter depth, i.e. 80 centimeters from the water surface. Computed concentration errors in 20, 40, 60 and 80 centimeter from the water surface for bottle sampling were 65.63, 56.13, 45.63 and 32.73 percent and for pipette sampling were 65.48, 57.02, 50.88 and 43.67 percent respectively. Also, there were no significant difference between sampling from center and side of the tanks and between mixing periods of 1, 2 and 5 minutes at 1% level of probability. Keywords: Sampling, Concentration, Sediment, Erosion plot, Runoff and Sediment Tank Introduction Soil as one of the basic factors in plant growth, is a fundamental national resource for human needs. In ideal conditions and applying appropriate agricultural rules, it takes about 300 years for generation of 2.5 centimeter of soil which is equal to 12.5 t/ha every year (Bybordi, 1993). Growing population rate and more need for food productions have destructive effects on natural resources causing soil degradation. Conserving generated soil and preventing soil loss need investigating soil erosion and studying sediment yield. The simplest method for studying the management of soil erosion is using soil erosion plots that lead to solve soil erosion and sediment related problems (Mutchler et al., 1994). Erosion plots provide such condition that a factor could be investigated while keeping constant all other factors. These plots are used in small size of one square meter for investigating interill erosion and big size of 11-198 meter long and 2-46 meter width for investigating both rill and interill erosions (Toebes and Ouryvaev, 1970). In small plots, runoff and sediment are collected in a tank, but in big plots a small portion of them are Proceedings of The Fourth International Iran & Russia Conference 1246 sampled. Although, developed methods like photogrametry (Cook and Valentine, 1979) and Laser scanner (Flanagan et al., 1995) are used in some researches, sampling by bottle is more common due to its simplicity and least costs. Usually, researchers pay attention on site selection and construction of erosion plots. But the main sources of error which are related to sampling method are not considered. Based on Bagnold’s works (explained by Shfaee Bajestan, 1994), in turbulence flow, sediment particles stay in suspended conditions, where vertical velocity be more than particle’s falling velocity. Point sampling by standard bottles is used for sampling suspended load in rivers (Przedwojski et al., 1995). Using bottles is common when sampling from erosion plot tanks but, they did not show accurate results (Lang, 1992). Zobisch et al., (1996) found significant difference among 5 sampling field staff results. They noticed that the depth of sampling, the velocity of mixing, and experience of samplers are the most important sources of error. To minimize sampling error, a cylindrical runoff and sediment sampler was made in Soil Conservation and Watershed Management Research Institute (SCWMRI) in Iran. It takes a column of water and accompanied suspended sediment from the whole depth of tank (Nikkami, 2002). Three different sampling instruments of bottle, pipette, and cylindrical sampler made in SCWMRI are compared in this research. Materials and methods Bottle, pipette, and cylindrical sampler were compared for their accuracy on sampling runoff and sediment from erosion plot tanks. Bottle sampling is usually used in erosion plot projects. Their volume differs from 0.5 to 4 liter based on their availability. As their volume increase, the time of sampling increases and more particles will settle in the tank and the accuracy would be affected. In this research, four 330 cc bottles were installed on a vertical metal bar with 30 degree angle and 20 cm far from each other (Nikkami et al., 2004). The bottle taps were opened simultaneously when located in the tank and took samples from 20, 40, 60 and 80 cm from the water surface (Fig. 1). Pipette as the second sampling instrument was used for sampling from the same depths. As shown in Fig. 2, four plastic tubes with 7 mm diameter were installed on a wooden bar with a vertical distance of 20 cm from each other. The another end of these four tubes were interred to one liter bottles which their taps were closed tightly and another tube from those bottles were connected to a 4 to 1 terminal and then with a tube from terminal to a suction (Nikkami et al., 2004). Cylindrical sampler made in SCWMRI, was the third sampler in this research. This sampler was designed to take a column of runoff and sediment from erosion plot tanks and it was consumed that the mixing speed and the amount of technician’s experience does not affect on sampling result (Nikkami, 2002). As illustrated on Figures 3 and 4, this sampler consists of three main parts. 1. Base unit and central shaft: Base unit consists of a 6.5 cm round pan that has a 5 cm diameter hole at the middle. The central shaft with 120 cm height is connected to the center of base unit. 2. Gauged cylindrical unit: Cylindrical unit is made by Plexiglas with 100 cm height and 5 cm diameter. This unit has been gauged to show the volume of sample and height of the collected runoff in the tank. 3. Upper unit: This part has been designed to fasten two other parts together and keep the sample to be replaced. Proceedings of The Fourth International Iran & Russia Conference 1247 Usually, a wooden steak is used to mix the runoff and sediment in plot project tanks. In this research, a wooden shovel has been used for this reason to keep the mixing condition constant in the whole research. The 1, 2 and 5 minute mixing period treatments were considered. To have sediment characteristics similar to one in plot projects, one ton of sediment was collected from the hill slopes of Sohrain-Gharachrian flood spreading station in Zanjan province with 40, 40 and 20 percent of sand, silt, and clay, respectively. Comparing grain size of collected sediment with 18 sediment samples from erosion plot tanks of this station showed no significant difference. The sediment has been mixed completely and spread on a lateral surface with 5 cm height. After drying at room temperature, samples of 400, 1000 and 2000 gr were prepared for making appropriate concentrations. The concentrations were 1.87, 4.68 and 9.36 gr/l which were similar to 75% of concentrations in erosion plot tanks of above mentioned station (Nikkami et al., 2004). The research was executed in following steps. 1. Preparing 1.87 gr/l sediment concentration in three 213.5 liter tanks 2. Mixing tank content clock wise with wooden shovel at 1, 2, and 5 minutes periods 3. Sampling 162 times with bottles, pipette, and cylindrical samplers from the center and side of the tanks. Based on one concentration, 3 mixing periods, 2 sampling location, and 3 replications, 18, 72, and 72 samples were taken by cylindrical sampler, bottles, and pipettes respectively. Because of low volume, 3 replications of bottle samples were mixed together. Therefore, samples of this step changed from 72 to 24 and the total samples from 162 to 114. 4. Computing sediment concentration of samples by passing them through 2.5 µm filters and drying in the oven and considering water volume of samples. 5. Preparing the sediment concentrations of 4.68 and 9.36 gr/l in the tanks 6. Mixing tank content clock wise with wooden shovel at 1, 2, and 5 minutes periods 7. Sampling with cylindrical sampler from the center of the tanks. Based on two concentrations, 3 mixing periods, 1 sampling location, and 3 replications, 18 samples were taken.
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