SAMPLING ACCURACY OF PIT VS. STANDARD RAIN GAGES ON THE FERNOW EXPERIMENTAL FOREST1

ABSTRACT: Catch in standard (unshielded) rain gages exposed 3 feet above the land surface was compared with catch in pit (buried) gages exposed 1 inch above the land surface. These tests confirmed that catch in standard gages under estimates point rainfall in forest openings, as well as in conventional weather stations. Pit gages caught significantly (P=0.05) more rain than did standard gages at each of four locations tested. Catch increases ranged from 2.3 to 3.4 percent.     

RAINGAGE NETWORK DESIGN USING NEXRAD PRECIPITATION ESTIMATES1

ABSTRACT: A general framework is proposed for using precipitation estimates from NEXRAD weather radars in raingage network design. NEXRAD precipitation products are used to represent space time rainfall fields, which can be sampled by hypothetical raingage networks. A stochastic model is used to simulate gage observations based on the areal average precipitation for radar grid cells. The stochastic model accounts for subgrid variability of precipitation within the cell and gage measurement errors. The approach is ideally suited to raingage network design in regions with strong climatic variations in rainfall where conventional methods are sometimes lacking. A case study example involving the estimation of areal average precipitation for catchments in the Catskill Mountains illustrates the approach. The case study shows how the simulation approach can be used to quantify the effects of gage density, basin size, spatial variation of precipitation, and gage measurement error, on network estimates of areal average precipitation. Although the quality of NEXRAD precipitation products imposes limitations on their use in network design, weather radars can provide valuable information for empirical assessment of rain‐gage network estimation errors. Still, the biggest challenge in quantifying estimation errors is understanding subgrid spatial variability. The results from the case study show that the spatial correlation of precipitation at subgrid scales (4 km and less) is difficult to quantify, especially for short sampling durations. Network estimation errors for hourly precipitation are extremely sensitive to the uncertainty in subgrid spatial variability, although for storm total accumulation, they are much less sensitive.     

PRECIPITATION MEASUREMENT BIASES IN THE UNITED STATES1

ABSTRACT: Gage-induced biases in monthly precipitation are estimated and removed at 1818 stations across the continental United States from 1950 through 1987. Deleterious effects of the wind and wetting losses on the interior walls of the gage were considered. These “corrected” estimates were obtained using site-specific information including wind speed, shelter-height air temperature, gage height, and sheltering. Wind speed and air temperature were interpolated at stations for which these data were not available using a spherically-based, nearest neighbor interpolation procedure. Results indicate that, as expected, biases are greater in the winter than the summer owing to the increased problems (particularly wind-induced) of measuring snowfall. In summer, percent errors range between 4 and 6 percent over nearly three-quarters of the United States with slightly larger errors over the Rocky Mountains. By contrast, winter biases are highly correlated with snowfall totals and percentage errors increase poleward, mimicking patterns of snowfall frequency. Since these biases are not trivial, they must be accounted for in order to obtain accurate and reliable time-series. If these biases are not properly addressed, serious errors can be introduced into climate change, hydrologic modeling, and environmental impact research.     

UNCERTAINTIES IN ESTIMATING THE WATER BALANCE OF LAKES1

ABSTRACT: Evaluation of hydrologic methodology used in a number of water balance studies of lakes in the United States shows that most of these studies calculate one or more terms of the budget as the residual. A literature review was made of studies in which the primary purpose was error analysis of hydrologic measurement and interpretation. Estimates of precipitation can have a wide range of error, depending on the gage placement, gage spacing, and areal averaging technique. Errors in measurement of individual storms can be as high as 75 percent. Errors in short term averages are commonly in the 15-30 percent range, but decrease to about 5 percent or less for annual estimates. Errors in estimates of evaporation can also vary widely depending on instrumentation and methodology. The energy budget is the most accurate method of calculating evaporation; errors are in the 10–15 percent range. If pans are used that are located a distance from the lake of interest, errors can be considerable. Annual pan-to-lake coefficients should not be used for monthly estimates of evaporation because they differ from the commonly used coefficient of 0.7 by more than 100 percent. Errors in estimates of stream discharge are often considered to be within 5 percent. If the measuring section, type of flow profile, and other considerations, such as stage discharge relationship, are less than ideal errors in estimates of stream discharge can be considerably greater than 5 percent. Errors in estimating overland (nonchannelized) flow have not been evaluated, and in most lake studies this component is not mentioned. Comparison of several lake water balances in which the risdual consists solely of errors in measurement, shows that such a residual, if interpreted as ground water, can differ from an independent estimate of ground water by more than 100 percent.     

A LARGE ELECTRONIC WEIGHING LYSIMETER SYSTEM: DESIGN AND INSTALLATION1

ABSTRACT: To measure crop evapotranspiration, a large double tank, electronic weighting lysimeter system was designed and installed at the Shahid Bahonar University farm, Kerman, Iran. The system was installed in a 50 m² underground building. It includes two tanks of 3.00 m in diameter and 1.75 m deep. The weighing mechanism for each tank is a set of three compression strain gage load cells, which are fixed on 1.20 m height column above the floor. According to the specification of the load cells, the maximum possible weighing error may be about 0.01 percent of total mass, which is equivalent to 0.28 mm of water, but the measured error was equal to 1 kg mass, which is equivalent to 0.14 mm of water. The load cell data from each tank and the on‐site environmental data (temperature, humidity, and wind velocity and direction) are automatically recorded and saved in a personal computer hard disk for further use and analysis.     

GAGE DENSITY AND LOCATION FOR ESTIMATING MEAN ANNUAL PRECIPITATION IN MOUNTAINOUS AREAS1

ABSTRACT: Data from a network of 45 shielded precipitation gages on the Reynolds Creek Experimental Watershed in Southwestern Idaho were analyzed to determine the optimum gage density for estimating mean annual precipitation. Four subsets of the 45 gage network were used to derive a curve of mean annual precipitation versus number of gages with a confidence band at the 95 percent level. When less than 20 gages were used in the estimate, the confidence interval widens rapidly. Estimates were improved by stratifying gages on the basis of plant cover class or by elevation bands. Sixty-four percent of the variation in mean annual precipitation was accounted for by elevation and cover class. The aspect and hydrologic soil classification were not statistically significant.     

FOG DRIP IN THE BULL RUN MUNICIPAL WATERSHED,OREGON1

ABSTRACT: Net precipitation under old growth Douglas fir forest in the Bull Run Municipal Watershed (Portland, Oregon) totaled 1739 mm during a 4Cbweek period, 387 mm more than in adjacent clearcut areas. Expressing data on a full water year basis and adjusting gross precipitation for losses due to rainfall interception suggest fog drip could have added 882 mm (35 in) of water to total precipitation during a year when precipitation measured 2160 mm in a rain gage in a nearby clearing. Standard rain gages installed in open areas where fog is common may be collecting up to 30 percent less precipitation than would be collected in the forest. Long term forest management (Le., timber harvest) in the watershed could reduce annual water yield and, more importantly, summer stream flow by reducing fog drip.     

HYDROLOGIC BUDGET FOR A FRESHWATER MARSH IN FLORIDA1

ABSTRACT: Accurate water balance calculations are essential for water resource and environmental management decisions, but many of the terms used in the equation are difficult to measure. In this study, a method for measuring rates of evapotranspiration and net seepage from a freshwater marsh in southwest Florida is described. The results are compared to evaporation pan estimates as well as to calculations that balanced all the terms in the hydrologic budget. The measured rates of evapotranspiration showed a. distinct seasonal trend ranging from an average high of 0.24 in/d during July 1992 to a low of 0.06 in/d in January 1993. Evapotranspiration rates were higher than Class A evaporation pan measurements during July and August, indicating transpiration by plants exceeded evaporation by pans. Net ground water seepage flowed out of the marsh except during periods of high water table conditions. When all terms in the hydrologic budget were evaluated, the equation balanced on a yearly basis with an error of 2 percent, on a seasonal basis with errors less than 7 percent, but on a monthly basis errors were as great as 30 percent. Total annual rainfall on the marsh was 45 percent of the total marsh hydrologic input and was approximately equal to the loss by evapotranspiration of 41 percent.     

长沙市灰霾天气与气象因子相关性研究

根据长沙市2013年PM2.5的逐日质量平均浓度、气象地面和高空观测数据,采用SPSS方法,分析了长沙市灰霾天气发生与气象因子的关系。结果表明:长沙市区的灰霾日以西北风向为主,PM2.5浓度与风速、降水呈显著负相关,与相对湿度、大气压、平均气温相关不明显。风速越小越不利于大气污染物的扩散,在没有降水的情况下,风速达到3.5m/s以上,空气质量才有好转;弱降水对污染物的浓度不会有明显的影响,降水量在5mm以下时,污染物的浓度不会有明显的下降,但强降水对空气有净化作用明显,在不同季节,不同时段,不同天气形势下降水的稀释作用不同;长沙秋冬季边界层稳定性几率高达80%以上,这种稳定层结构是长沙市区各种大气污染源不易扩散的重要因素之一。     

SPACE-TIME VALIDATION OF A THUNDERSTORM RAINFALL MODEL1

A probability model for predicting the occurrence and magnitude of thunderstorm rainfall developed in the southwestern United States was tested in the metropolitan Chicago area with reasonable success, especially for the moderate to the extreme runoff-producing events. The model requires the estimation of two parameters, the mean number of events per year and the conditional probability of rain given that an event has occurred. To tie in the data from more than one gage in an area, an event can be defined in several ways, such as the areal mean rainfall exceeding 0.50 inch and at least one gage receiving more than 1.0 inch. This type of definition allows both of the model parameters to be obtained from daily warm-season rainfall records. Regardless of the definition used a Poisson distribution adequately described the number of events per season. A negative binomial distribution was derived as representing the frequency density function for rainfall where several gages are employed in defining a storm. Chicago data fit both distributions very well at events with relatively high return periods. The results indicate the possibility of using the model on a regional basis where limited amount of data may be used to estimate parameters for extensive areas.     

EVALUATION OF EROSION CONTROL PRODUCTS WITH AND WITHOUT ADDED POLYACRYLAMIDE1

ABSTRACT: Polyacrylamide (PAM) has been demonstrated to greatly reduce erosion in furrow irrigation, but much less is known about its effectiveness on the much steeper slopes typical of construction sites. The purpose of this study was to determine if anionic PAM would enhance erosion control either alone on bare soil or in combination with four types of ground covers commonly used for grass establishment: straw, straw erosion control blanket (ECB), wood fiber, and mechanically bonded fiber matrix (MBFM). Tests were conducted under natural rainfall and vegetation on a 4 percent slope (bare soil, straw, ECB, and MBFM) or using a rainfall simulator (bare soil, straw, wood fiber, MBFM) on either 10 percent or 20 percent slope on three different soil substrates. All ground cover treatments were evaluated with and without PAM applied in solution at 19 kg/ha. The straw, ECB, and MBFM significantly reduced runoff volume, average turbidity, and total sediment lost over five rainfall events on the vegetated plots. The addition of PAM to ground covers only occasionally had significant effects on runoff parameters but did significantly increase vegetative coverage overall. The rainfall simulator tests produced similar results after four events, with the straw, wood fiber, and MBFM all having significantly lower turbidity than the bare soil. The PAM significantly reduced turbidity for both the first and second events but did not consistently improve runoff quality after multiple rainfall events for any ground cover‐soil combinations tested. Separate tests of PAM applied before or after straw did not indicate a clear advantage of either approach, but runoff turbidity was often significantly reduced with PAM, especially at the 20 percent slope. Turbidity reductions were attributed to flocculation of eroded sediment.     

PUMP SYSTEM FOR IMPROVING RUNOFF RECORDS DURING WINTER1

ABSTRACT: Freezing winter temperatures can cause icing of outdoor weirs used to measure surface runoff. Ice typically forms in the notch and on the crests of short-crested V-notch weirs, causing incorrect gage heights to be measured. A method for reducing the effects of ice formation on weirs and weir pools using a pump is presented and evaluated. Warmer water from the bottom of the weir pool is pumped to the surface, reducing the opportunity for the water surface to freeze. The pump is shown to work except under extremely cold conditions, improving runoff records from 27 percent to 60 percent. The pump system has no practical effect on measured gage height. Frequency distributions of flow rates and air temperatures under measured ice-free and other weir conditions are presented. Suggestions for use of the pump system under temperature conditions other than those in this study are given.     

RUNOFF,EROSION, AND SOLUTES IN THE LOWER TRUCKEE RIVER,NEVADA, DURING 19691

The Truckee River heads in the Sierra Nevada at Lake Tahoe, and terminates in Pyramid Lake. During the 1969 water year, flow about 9 miles upstream from the mouth (974,000 acre-ft) was almost four times the long-term average, due mainly to heavy winter rains and spring snowmelt. A short period of low-altitude rainfall produced the highest concentrations of suspended sediment, whereas a much longer subsequent period of snowmelt yielded a much greater total quantity of material. The upper 90 percent of the basin yielded about 260 acre-feet (630,000 tons) of sediment at the Nixon gage, whereas an estimated 2,800 acre-feet (6.8 million tons) was contributed by erosion of about 200 acres of river bank below the gage. Solute content at the gage ranged from 80 to 450 mg/l, dominated by calcium, sodium, and bicarbonate, plus silica in the most dilute snowmelt and chloride in the most concentrated low flows. Solute load totaled about 130,000 tons, of which the principal constituents in Pyramid Lake-sodium plus equivalent bicarbonate and chloride-amounted to almost 40,000 tons. The total solute load during a year of average flow may be 45,000-55,000 tons, including 18,000-22,000 tons of principal lake constituents.     

WIND DRIVEN RAIN DISTRIBUTIONS AROUND STREET CANOPIES1

ABSTRACT: Wind driven raindrop tracking is used to investigate the microscale redistribution of wind driven rainfalls in street canopies by combining a Eulerian wind flow model and a Lagrangian raindrop tracking model. The former conducts large eddy simulations of the turbulent flows in street canopies, and the latter performs raindrop trajectory calculations by releasing a large number of raindrops into the computational domain. The wind speed model is verified with available wind tunnel measurement. Twenty sets of simulations are carried out for various building configurations and driving rain angles. The simulated results show that the trajectories of smaller raindrops are more slanting and more influenced by the multibuilding perturbed flow field. Impingement of raindrops on the building envelope increases from bottom to top. The height of the front building is a significant factor affecting wind driven rain redistribution. Distinct nonuniform spatial rainfall distributions are found for scenarios with high building configurations and low driving rain angles. The simulated results are further integrated to assess the effect of real raindrop size distributions by weighing the volumetric fraction of a range of drop sizes. There is about 10 percent variation in spatial extent of street canopies. An overall 5 to 17.4 percent increase of the rainfall amount in the upwind zone is observed.     

THROUGHFALL IN PLANTED STANDS OF FOUR SOUTHERN PINE SPECIES IN EAST TEXAS1

Throughfall was measured during 1978–79 beneath the canopies of adjacent stands of four major southern pine species, all on identical soil type and topography in the Stephen F. Austin Experimental Forest. Observations from 44 storms in a randomized network of 15, 5.08 cm PVC gages in a 0.4 ha plot of each species showed that throughfall expressed as percent of storm precipitation, is greatest under longleaf pine and least under loblolly pine; throughfall under shortleaf and slash pine did not differ significantly. Generally, through-fall decreased with storm size and intensity, with distance from the nearest tree stem, and is greater during summer half-year (May–October). Canopy drips, apparently accounting for the greater throughfall for the gage position closer to the stems, were more numerous than reported elsewhere. The 5.08 cm PVC gages proved to be acceptable substitutes for standard nonrecording gages in measuring throughfall. A network of 15 such gages was sufficient to sample throughfall data with 90 percent accuracy in each of the four southern pine plantations.     

Climate variability,water levels,and fish catch in the Upper East Region of Ghana

The aim of the research was to investigate the effects of climate variability on selected water bodies in the Upper East Region of Ghana using time series decomposition and simple linear regression analyses. Data on temperature and rainfall (1960–2015), annual total fish catch (1996–2016), and the recorded water levels (1987–2015) of a major reservoir, the Tono, were used. Time series decomposition analyses were performed on the rainfall, temperature, and water level data to identify their trends. While temperature was increasing, rainfall was decreasing and resulted in a decrease in the water level in the Tono Reservoir. The decreasing water level in the reservoir made fish catch easier, which led to overfishing. Out of the other 39 dugouts studied, 8 (21%) were silted. Also, rainfall was decreasing at 4.4% per decade and minimum and maximum temperatures were increasing at 2.5% and 0.03% per decade, respectively. The minimum, maximum, and mean water levels of the Tono Reservoir were 3.7, 8.0, and 4.9 meters (m), respectively. The water level of the Tono Reservoir was decreasing by ?0.08 m per year. It is concluded that the water level in the Tono Reservoir was continually decreasing as a result of decreasing rainfall and increasing maximum and minimum temperatures. To maintain a much more stable microclimate and decrease the siltation rate of the reservoirs, farmers are advised to stop farming along the banks of water bodies and avoid clearing vegetation. Fishermen are also encouraged to adopt fish farming in enclosed areas within the reservoir to meet the growing protein demands in the Upper East Region of Ghana.     

ASSESSMENT OF SAMPLING ERROR ASSOCIATED WITH SOIL MOISTURE ESTIMATION DESIGNS1

A spectral formalism was developed and applied to quantify the sampling errors due to spatial and/or temporal gaps in soil moisture measurements. A design filter was developed to compute the sampling errors for discrete measurements in space and time. This filter has as its advantage a general form applicable to various types of sampling design. The lack of temporal measurements of the two‐dimensional soil moisture field made it difficult to compute the spectra directly from observed records. Therefore, the wave number frequency spectra of soil moisture data derived from stochastic models of rainfall and soil moisture were used. Parameters for both models were estimated using data from the Southern Great Plains Hydrology Experiment (SGP97) and the Oklahoma Mesonet. The estimated sampling error of the spatial average soil moisture measurement by airborne L‐band microwave remote sensing during the SGP97 hydrology experiment is estimated to be 2.4 percent. Under the same climate conditions and soil properties as the SGP97 experiment, equally spaced ground probe networks at intervals of 25 and 50 km are expected to have about 16 percent and 27 percent sampling error, respectively. Satellite designs with temporal gaps of two and three days are expected to have about 6 percent and 9 percent sampling errors, respectively.     

SEDIMENT TRANSPORT IN FLOW DISTURBED BY RAINFALL1

Studies were conducted in a closed system recirculating research flume to evaluate the relative effects of high intensity rainfall on von Karman's universal constant and the sediment transport capacity of shallow flow. The tests in this study were conducted at flow depths of 0.3 ft and less with discharges less than 0.5 cfs. The point velocities in the flow were determined with a Pace CD-25 pressure transducer and an inclined manometer connected in parallel to a Pitot-static tube of the standard Prandtl design. Regression analyses were performed on the velocity data to determine the best fit dimensionless velocity curve on semilogarithmic paper. Von Karman's universal constant was then evaluated from the slope of the regression line. Point sediment samples were siphoned from the flow with a stainless steel-pipette sediment sampler. Sediment concentrations were found with a filtering technique. Sediment samples were taken with and without rainfall to evaluate the relative effect of the rainfall on the transport capacity of shallow flow.     

EFFECT OF RAINFALL EXCESS CALCULATIONS ON MODELED HYDROGRAPH ACCURACY AND UNIT-HYDROGRAPH PARAMETERS1

ABSTRACT: Two methods of computing rainfall excess in the U.S. Army Corps of Engineers’flood hydrograph package (HEC-1), the Initial and Uniform method and the Exponential method, are compared to evaluate the effects on modeled hydrograph accuracy. Two computed unit-hydrograph parameters, time of concentration and storage coefficient, were also compared. Rainfall and runoff data from 209 storms in 32 gaged basins in Illinois were used to calibrate the HEC-1 model. Three hydrograph characteristics - sum of incremental flows, peak discharge, and time of peak discharge - were used to evaluate modeled hydrograph accuracy. Mean percent error for each basin and hydrograph characteristic was computed. An evaluation of the mean errors indicates that, although some bias in modeled hydrograph accuracy is evident, rainfall excess computed using either method results in a computed hydrograph accuracy that is within generally accepted limits. Application of a linear-regression model shows no significant differences in computed values of unit-hydrograph parameters.     

HYDROLOGY OF A DRAINED FORESTED POCOSIN WATERSHED1

ABSTRACT: In order to assess the effects. of silvicultural and drainage practices on water quality it is necessary to understand their impacts on hydrology. The hydrology of a 340 ha artificially drained forested watershed in eastern North Carolina was studied for a five-year period (1988–92). Effects of soils, beds and changes in vegetation on water table depth, evapotranspiration (ET) and drainage outflows were analyzed. Total annual outflows from the watershed varied from 29 percent of the rainfall during the driest year (1990) when mostly mature trees were present to as much as 53 percent during a year of normal rainfall (1992) after about a third of the trees were harvested. Annual ET from the watershed, calculated as the difference between annual rainfall and outflow, varied from 76 percent of the calculated potential ET for a dry year to as much as 99 percent for a wet year. Average estimated ET was 58 percent of rainfall for the five-year period. Flow rates per unit area were consistently higher from a smaller harvested block (Block B - 82 ha) of the watershed than from the watershed as a whole. This is likely due to time lags, as drainage water flows through the ditch-canal network in the watershed, and to timber harvesting of the smaller gaged block.