The adsorption of soluble P by surface soil and suspended sediment material during transport in surface runoff under field and simulated laborato ry conditions was investigated. The soluble P concentration of surface runoff from several Southern Plains cropped and grassed watersheds decreased with an increase in sediment concentration. A linear inverse relationship between soluble P and sediment concentration was significant over a wide range in sediment concentration. The slope values of the relationship were similar for different watersheds on the same major soil type. Using soil from these watersheds in simulated surface runoff, sorption of soluble P added in rainfall was found to occur during transport. The magnitude of this sorption was more closely related to the sorptive capacity of the sediment in the surface runoff than to that of the surface soil material. The results suggest that for unfertilized watersheds and for watersheds where fertilizer P is incorporated into the surface soil, away from the zone of immediate removal in surface runoff, the leaching of P from the vegetative cover can contribute significant amounts of soluble P to runoff, and that soil material may act as a P sink rather than a P source.

Seven cropland watersheds and four rangeland watersheds in central Oklahoma were monitored for surface hydrology and discharge of nitrogen, phosphorus, and sediment over a 1 year period. Precipitation and runoff were much above normal during the study. Sediment losses from the continuously grazed rangeland watersheds ranged from 18 to 23 metric tons/ha during the study. None of the sediment losses from the other watersheds exceeded 10 metric tons/ha.Total nutrients discharged in runoff ranged from 2 to 15 kg/ha of N and 1 to 11.5 kg/ha of P. Flow‐weighted mean concentrations ranged from 1 to 6 ppm of total N, 0.2 to 1.9 ppm of nitrate‐N, 0.5 to 4.8 ppm of total P, and 0.04 to 0.9 ppm of soluble P. Runoff losses of soluble inorganic nitrogen were generally less than those quantities received in rainfall. Concentrations of soluble phosphorus in runoff from the cropland watersheds were much greater than from the rangeland watersheds. Losses of fertilizer nitrogen and phosphorus did not exceed 5% of the most recent applications, although surface runoff was 4‐ to 10‐fold greater than that observed in previous years.

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No abstract

Nitrogen and phosphorus discharges in runoff from nearly level cropland and 3% sloping rangeland were measured from July 1972, to June 1976. Sediment discharges and runoff amounts from these 5‐ to 18‐ha watersheds were measured from July 1966, to June 1976. Sediment and nutrient discharges varied greatly from year to year and between different land uses. We concluded that long records are needed to compare discharges from different management practices. The average and maximum annual sediment discharges, respectively, were 3,600 and 8,900 kg/ha from irrigated cotton (Gossypium hirsutum), 900 and 3,900 kg/ha from dryland wheat (Triticum aestivum), 400 and 1,800 kg/ha from range with limited grazing, and 9,000 and 23,000 kg/ha from overgrazed range. Maximum annual sediment discharges occurred during the period in which nutrient discharges were measured. Maximum annual nutrient discharges were 13 kg/ha total N, 4 kg/ha nitrate N, 11 kg/ha total P, and 2 kg/ha soluble P. The average annual discharge for each nutrient form and land use was about half of its maximum value. Nitrate accounted for 10 to 30% of the total N discharged. Soluble phosphate accounted for about 20% of the total P discharged from cropland, and <10% of that discharged from rangeland. Annual deposition in rainfall averaged 5 kg/ha N and 0.15 kg/ha P.

Large area soil moisture estimations are required to describe input to cloud prediction models, rainfall distribution models, and global crop yield models. Satellite mounted microwave sensor systems that as yet can only detect moisture at the surface have been suggested as a means of acquiring large area estimates. Relations previously discovered between microwave emission at the 1.55 cm wavelength and surface moisture as represented by an antecedent precipitation index were used to provide a pseudo infiltration estimation. Infiltration estimates based on surface wetness on a daily basis were then used to calculate the soil moisture in the surface 0–23 cm of the soil by use of a modified antecedent precipitation index. Reasonably good results were obtained (R2= 0.7162) when predicted soil moisture for the surface 23 cm was compared to measured moisture. Where the technique was modified to use only an estimate of surface moisture each three days an R2 value of 0.7116 resulted for the same data set. Correlations between predicted and actual soil moisture fall off rapidly for repeat observations more than three days apart. The algorithms developed in this study may be used over relatively flat agricultural lands to provide improved estimates of soil moisture to a depth greater than the depth of penetration for the sensor.

Amounts of nitrogen, phosphorus, and sediment were measured in runoff from grassland watersheds in the Blackland Prairies, High Plains, Reddish Prairies, and Rolling Red Plains land resource areas of Oklahoma and Texas. Periods of study were 3 to 5 years and included treatments involving fertilization, cultivation, and burning. Overall nutrient concentrations generally ranged from 2 to 10 mg/l for nitrogen and 0.3 to 2 mg/l for phosphorus. In most cases, less than half the nutrients existed as soluble forms in the runoff water. Typically, annual sediment losses were less than 0.5 metric tons/ha. Corresponding losses for nitrogen and phosphorus were less than 5 and 2 kg/ha, respectively. In the case of nitrate, more was received in precipitation than was lost in runoff. Total nitrogen and phosphorus losses were strongly correlated with sediment losses. Preliminary results using predictive techniques to estimate nutrient and sediment discharge from the watersheds were encouraging. With proper management, the likelihood of any adverse environmental effects due to nutrient and sediment discharge from Southern Plains grasslands appears slight. Increased demand for agricultural products is causing more intensive use of Southern Plains grasslands. Such use includes concentrated grazing, fertilization, and weed/ brush control. In addition, a considerable area of marginal land shifts from grassland to cropland and vice-versa depending on wheat and cattle prices. In Oklahoma and Texas, the major portion of the private pasture and rangeland is listed by the Soil Conservation Service as requiring some kind of conservation treatment (USDA 1965). Thus, many of the grasslands are fragile, and studies to assess the potential seriousness of erosion and water quality hazards associated with grassland practices are needed. Discharge studies of nutrients and sediment from grassland watersheds (

Four native grassland watersheds were monitored for nitrogen and phosphorus nutrient losses in surface runoff. The watersheds, paired in surface hydrology and grazing management, were 8 to 11 ha in area with 3% slopes. One watershed of each pair was fertilized with 85 kg N/ha (as NH4+‐N) and 75 kg P/ha surface broadcast. Ten 3.66 by 5.49 m (20 m2) areas within each watershed were covered with plastic sheets during fertilization to provide unfertilized check plots. Fertilizer losses in surface runoff over the first year were 5% or less of the amounts applied. Soluble NH4+‐N concentrations in surface runoff increased significantly only during the first month after fertilizer application. Soluble P concentrations increased sharply after fertilization and remained relatively high even after 12 mo. Forage yields were increased 50 to 100% on fertilized areas. Increases in nutrient uptake of fertilized vegetation the first year equaled 17 to 25.5 kg/ha and 7.5 to 11 kg/ha of the added N and P, respectively.