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Cevdet Köse 1 , Alaettin Taysun 2 1 Köy Hizmetleri Araştırma Enstitüsü, Menemen-İZMİR, 2 Ege Üniversitesi Fen Bilimleri Enstitüsü, Bornova-İZMİR ABSTRACT This research was carried out to determine P factor of USLE under the contour tillage applications using rainfall simulators on Gediz Basin located in Aegean Region. 70 mm/h for total 90 minutes of rainfall was applied to each 7.30 m by 1.70 m sized total 10 parcels using computer controlled rainfall simulator, which has Veejet 80100 type nozzles. Simulated rainfall was applied 60-minute duration on dry soil (dry rain) first time and after 30 minutes 30-minute duration rainfall applied on wet soil (wet rain).According to the results, runoff beginning time in dry rainfall condition, has been higher than wet rainfall condition. Most of the loss of soil has been observed in winter and spring whereas the least lost has been observed in that summer rainfall. Increase of tillage depth and stone cover, decreases the soil loss. The factor the Universal soil loss equation for contour tillage has been determined. considering different soil properties as well as the seasonal affects. Factor values have changed at a range of 0.06 and 0.97. INTRODUCTION Effective control of soil erosion requires ability to quantitatively to predict the amount of soil loss, which would occur under alternate management strategies and practices. The Soil- erodibility factor (K) and the conservation practice factor (P), which are the factors of Universal soil loss equation that is widely used in predicting soil loss, needs to be determined in different soil groups and climate conditions. But, since it is expensive and time consuming to measure the run off under the natural conditions, it is preferred to determine these factors through the rainfall simulator, which enables the scientist to carry out these studies at the many locations and within the short time period. In this study, using rainfall simulator, it has been determined parameter P of USLE equation and relationships between soil loss and some physical features of soils in Gediz Basin conditions. MATERIAL AND METHODS Study Area : The research was conducted in Gediz basin, which is located at west part of the Turkey. The study areas from which experimental soil samples are taken are placed on almost middle part of the Basin. Stone cover and surface roughness values and some properties of the study areas are given in table 2.1 and 2.2, respectively.   Some Features of Rainfall Simulator : In this study, the computer controlled rainfall simulator, with high kinetic energy, has six rainfall storm unit and V- jet 80100 type nozzles, was used (Taysun, 1985). Method : The experimental plot sized 1.70-m. width, 7.30-m length on 7, 9,13 per cent slopes. The simulated rainfall applications were replicated two times for each trial. The rainfall at 69.75-mm h-1 intensity was applied to dry soil condition during 60-minutes period, then 30 minutes period interval and 30- min rainfall was applied to wet soil, thus, totally 90- min rainfall was applied. Rainfall was applied to the plot number1, 2 and, 3 in winter, number 4 and 5 in spring, number 6, 7, 8, and 9 in summer and number10 in autumn. Simulation time was controlled and recorded by means of chronometer and computer. Run off samples were manually collected and soil loss were measured after each rainfall application. The soil conservation factor P was calculated as a ratio of soil losses from contour plots to losses from the plots that cultivated along with the land slope (Doğan and Güçer, 1976). RESULTS AND DISCUSSON The data obtained from the dry soil and wet soil trials were given in table 3.1 and 3.2 respectively.   Table 2 shows that factor P varied from 0.00 to 1,13 for dry soil conditions, while it ranged from 0.07 to o.61 in wet soil conditions. P value was zero in trial 8, which was one of the plots with dry soil condition and no soil loss occurred in this trial. On the other hand, the soil loss from the plot that cultivated up and down slope was 0.655 t/ha. It can be seen that, the large amount of soil loss has occurred in the trial 2, as a result of this, factor P was found as 1.13, whereas normally it is expected not to be bigger than one. Because of this, some of the contour furrows was damaged after16 minutes heavy rainfall. Factor P for the cumulative rainfall was determined between 0.06-0.97. The highest P value was obtained from trial 2, while the lowest one from trial 9. The reason of obtaining high P value is that the amount of soil loss from P plot is very close to those of the plot that cultivated parallel to land slope. On contrast to that, low P factor value shows that the amount of soil loss from plot-P was smaller than those of the plot that cultivated up and down slope. The high P value of plot 1and 6 is attributed to the high percentage of stone cover in these plots. CONCLUSIONS The following conclusions can be stated from this study: - After rainfall simulation to the plots, the time of concentration -Tc (Run off beginning time) was longer in the dry soil condition (60min) when compared to wet soil condition (30 min) - Negative correlation was determined between the time of concentration (Tc) and soil loss for wet soil condition. - It was determined that the time of concentration (Tc) in both conditions was longer in summer, compared to that in winter, spring and autumn. - The highest run off rate was obtained in winter and spring, while the lowest run off rate was in summer after dry, wet and cumulative rainfall implementation. The run off in winter caused 253 % more soil loss at the plots (number 1 and 6), which has the same soil features, compared to those in summer. - Under the dry soil condition, soil loss in summer were less than that in spring and winter, but under the wet soil condition, in contrast to that, soil loss in summer time was more than that in spring and winter time. As a result of experiment, cumulative rainfall caused 21 % more soil loss in summer than in winter. - Increasing of 4-5 cm in the depth of ploughing at cross -slope farming decreased run off and soil loss. - It was measured that soil loss at cumulative rainfall were less by 190 %, 45% and 87 % in February, March and July respectively. - As the stone cover on the soil surface increased, because of mulch effect, soil loss decreased. - The factor P, which is one of the important factors of Universal Equation, was determined according to different soil features and also seasonal differences. As it seen from the table 3.3, the P factor ranged between 0.06 and 0.97. - As increasing stone cover on the soil surface, P value also increased. In this case, contour cultivation has less effect on soil loss. REFERENCES Doğan, O. ve Güçer, C. 1976. Su erozyonunun nedenleri-oluşumu ve Üniversal denklem ile toprak kayıplarının saptanması. Merkez TOPRAKSU Araştırma Enstitüsü Müdürlüğü Yayınları, Genel Yayın No.41, Ankara. Taysun, A. 1977. Bornova ve civarındaki mevcut büyük toprak gruplarının, bazı fiziksel ve kimyasal özellikleri ile bunların erozyonla olan ilişkileri üzerinde araştırmalar (Doktora Tezi). E.Ü. Zir. Fak. Toprak Bölümü, Bornova. Taysun, A. 1982. Gediz Havzasının rendzina tarım topraklarında, yapay yağmurlayıcı yardımıyla, taşlar bitki artıkları ve polyvinilalkol'ün (PVA) toprak özellikleri ile birlikte erozyona etkileri üzerinde araştırmalar (Doçentlik Tezi). E.Ü.Zir.Fak. Toprak Bölümü, Bornova. Taysun, A. 1985. Doğal ve yapma yağışın karşılaştırılması, yağış benzeticiler ve damla düşme hızı tayin aletleri. Köy Hizmetleri Genel Müdürlüğü Menemen Araştırma Enstitüsü Yayınları, Genel Yayın No. 119, Menemen. Taysun, A., Uysal, H., Köse, C., Mollenhauer, K. ve Frede, H.G. 1996. Ege Bölgesinde toprak aşınabilirlilik faktörünün (K) yapay yağmurlayıcı (=Rainfall simulator) yardımıyla arazi koşullarında saptanması. Proje No. TOAG-859, Bornova TOPRAKSU. 1974. Gediz Havzası Toprakları. TOPRAKSU Genel Müdürlüğü Yayınları.No. 302, Ankara. U.S.Department of Agriculture. 1951. Soil survey manual. Agric. Handb.No. 18. |