Optical Spectropluviometer Research Articles

Detachment of soil organic carbon by rainfall splash: Experimental assessment on three agricultural soils of Spain

An experiment was undertaken to measure the concentration of soil organic carbon (SOC) in particles mobilized by rainfall splash under natural precipitation and to assess its relationship with soil and precipitation properties. Splash cups were deployed on three agricultural soils typical of the central Ebro Valley in Spain (a Cambisol, a Gypsisol, and a Solonchak), and the rainfall characteristics (intensity, kinetic energy) were measured by means of a disdrometer (optical spectro-pluviometer). Evidences of SOC enrichment, i.e. a significantly higher concentration in the splashed material with respect to the parent material, were found in the three soils under study. Differences were found, too, between two particle size fractions (less than 0.05mm and between 0.05 and 0.5mm), with higher SOC enrichment in the coarsest fraction. While the amount of splash was clearly related to the erosivity of each rainfall event, no significant effect was found with respect to the SOC concentration. Between the three soils, the Gypsisol exhibited the highest rates of SOC enrichment, and also the largest difference between size fractions. Splash plays an important role on mobilizing fresh carbon fractions, and under certain conditions it may interrupt the soil carbon cycling by favoring the removal of SOC by other erosive processes such as runoff wash, thus preventing its incorporation into the soil carbon pool.

Consistency in <i>Z</i>-<i>R</i> Relationship Variability Regardless Precipitating Systems, Climatic Zones Observed from Two Types of Disdrometer

Data from rain Drop Size Distributions gathered on five sites in Africa as well as those of the pilot site in Kourou (French Guyana, South America), located in different climatic zones, and collected by two types of disdrometer (the impact JW RD-69 disdrometer and the Optical Spectro-Pluviometer, OSP) are used to study the consistency of the reflectivity factor-rain rate at the ground (Z-R) relationship variability. The results clearly confirm that the relationship Z-R knows a large spatial variability, from a type of precipitation to another and within the same precipitation regardless the type of disdrometer used for DSD measurements. Base on the similarity of the relations reflectivity factor-rain rate and ratio median volume diameter over the total number of drops-rain rate, the variability of the Z-R coefficients (A, b) through the simultaneously implication of the size and number of drops which characterize the DSD was exhibited. It was shown that the relationships A-α and b-β designed to understand the involvement of parameters D0 and NT of DSD in the variability of the relationship Z-R are similar regardless the types of disdrometer used. However, the relations A-α in the Sahelian region appear to deviate from those of Guinean, equatorial and Soudanian zones. The plausible reasons were discussed.

Path-Average Rainfall Estimation from Optical Extinction Measurements Using a Large-Aperture Scintillometer

AbstractThe potential of a near-infrared large-aperture boundary layer scintillometer as path-average rain gauge is investigated. The instrument was installed over a 2.4-km path in Benin as part of the African Monsoon Multidisciplinary Analysis (AMMA) Enhanced Observation Period during 2006 and 2007. Measurements of the one-minute-average received signal intensity were collected for 6 rainfall events during the dry season and 16 events during the rainy season. Using estimates of the signal base level just before the onset of the rainfall events, the optical extinction coefficient is estimated from the path-integrated attenuation for each minute. The corresponding path-average rain rates are computed using a power-law relation between the optical extinction coefficient and rain rate obtained from measurements of raindrop size distributions with an optical spectropluviometer and a scaling-law formalism for describing raindrop size distribution variations. Comparisons of five-minute rainfall estimates with measurements from two nearby rain gauges show that the temporal dynamics are generally captured well by the scintillometer. However, the instrument has a tendency to underestimate rain rates and event total rain amounts with respect to the gauges. It is shown that this underestimation can be explained partly by systematic differences between the actual and the employed mean power-law relation between rain rate and specific attenuation, partly by unresolved spatial and temporal rainfall variations along the scintillometer path. Occasionally, the signal may even be lost completely. It is demonstrated that if these effects are properly accounted for by employing appropriate relations between rain rate and specific attenuation and by adapting the pathlength to the local rainfall climatology, scintillometer-based rainfall estimates can be within 20% of those estimated using rain gauges. These results demonstrate the potential of large-aperture scintillometers to estimate path-average rain rates at hydrologically relevant scales.

Measurement of raindrop size distributions in tropical rain at Costa Rica

This paper presents the results of a unique opportunity of analyzing a large drop-size-distribution (DSD) sample. Using a well calibrated Optical Spectro Pluviometer sensor for measuring 1-min DSDs, a sample of one year and seven months of tropical rain has been collected in San Jose, Costa Rica. In addition, a quality control procedure was applied to the sample in order to construct a very robust data set. We examined frequency distributions of some DSD-derived parameters (such as rainrate, reflectivity, liquid-water content, median and mean volume diameters). Distinct shapes of average DSDs are observed for convective, transition, and stratiform rain. It is found that the transition and the stratiform curves are parallel. In terms of drop growth, this suggests that stratiform and transition raindrops evolve by similar physical process. Stratifying the DSDs by precipitation types based on physical criteria, prior to normalization, leads to very distinct normalized shapes. Thus, the idea that all DSDs can be represented by a single two-parameter normalization does not apply in our case. Overall, the results indicate that many of the microphysical characteristics of the rain in the sample resemble the ones observed in orographic rain at Hawaii.

An Experimental Study of Small-Scale Variability of Radar Reflectivity Using Disdrometer Observations

Abstract Analysis of data collected by four disdrometers deployed in a 1-km2 area is presented with the intent of quantifying the spatial variability of radar reflectivity at small spatial scales. Spatial variability of radar reflectivity within the radar beam is a key source of error in radar-rainfall estimation because of the assumption that drops are uniformly distributed within the radar-sensing volume. Common experience tells one that, in fact, drops are not uniformly distributed, and, although some work has been done to examine the small-scale spatial variability of rain rates, little experimental work has been done to explore the variability of radar reflectivity. The four disdrometers used for this study include a two-dimensional video disdrometer, an X-band radar-based disdrometer, an impact-type disdrometer, and an optical spectropluviometer. Although instrumental differences were expected, the magnitude of these differences clouds the natural variability of interest. An algorithm is applied to ...

Sprinkler droplet size distribution measured with an optical spectropluviometer

In solid set sprinkler irrigation systems, many factors have influence on water partitioning efficiency and uniformity. Drop size formation in the irrigation process is affected by some of these factors. Soil crustiness and damage to bare soil structure are problems associated with large drops. This study shows the results of measured drop size distributions in medium-sized sprinklers. An optical spectropluviometer (OSP) with infrared beam was used to measure drop size distributions. A variety of factors were checked: sprinkler type, nozzles, jet straightening vane and working pressure. The results showed that working pressure has the most influence on drop sizes; under low-pressure conditions, drops with a diameter of about 9 mm can be formed. The influence of the jet straightening vane and the types of nozzles and sprinklers has also been studied. A good similitude is obtained between the medium drop diameters measured and those simulated with the SIRIAS ballistic simulation model.

Instrumental Uncertainties inZ–RRelationships and Raindrop Fall Velocities

Abstract This study demonstrates the sensitivity of reflectivity–rainfall rate (Z–R) relationships, which were derived from disdrometer-based drop size distribution measurements, to the fall velocity of the drops. The dataset used comes from the simultaneous observation of a series of five moderate rainfall events with a Joss and Waldvogel disdrometer and an optical spectropluviometer. The signal-processing software of the latter was able to measure the residence time of the drops in its sampling volume, enabling computation of the volumetric drop concentration without any assumption on the fall velocity of the drops. The Z–R relationships derived from the two instruments are significantly different. This difference is shown to essentially arise from the drop fall velocities in a twofold manner. First, it comes from the use of a theoretical terminal fall velocity in still air to compute the drop concentration from the drop flux. Second, it comes from the principle of drop size measurement by the impact-ty...

Statistical and physical analysis of soil detachment by raindrop impact: Rain erosivity indices and threshold energy

Laboratory experiments have been conducted to study the effects of various simulated rain properties on soil detachment due to raindrop impact. Two soil materials were tested: a fine sand and a silt loam. The mass of detached sediment was measured with the splash cup technique. The splash cups filled with soil material were exposed to simulated rain intensities ranging between 10 and 140 mm h−1. The detached sediment was collected and weighed, whereas rain intensity, drop diameter, and fall velocity of each raindrop were measured with an optical spectropluviometer. A statistical analysis has been made in order to evaluate which rain parameter best predicts the splash detachment rate. Linear fits between the rate of detached sediment and the product of drop size (D) and drop velocity (V), that is, DαVβ with values of α varying between 1 and 6 and values of β varying between 0 and 2, have been computed. The results indicate that values for α ranging between 3 and 6 and for β smaller than or equal to 2 best describe the rate of detached sediment. Although all indices containing the mass of water (i.e., D3) predicted splash detachment rates relatively well, the product of momentum and drop diameter (D4V) was slightly superior in describing splash detachment. The existence of a rain erosivity threshold (critical kinetic energy) to initiate soil detachment is confirmed. This threshold energy equals 5 μJ for the fine sand and 12 μJ for the silt loam and compares relatively well with the threshold energy data for other soils reported in the literature.

Instrumental Uncertainties inZ–RRelations

Abstract Intercomparisons among three different sensors for ground-based measurement of drop size distributions are presented: the Joss–Waldvogel distrometer, the optical spectro-pluviometer, and the precipitation occurrence sensor system. Data corresponding to stratiform, continental rain are analyzed, and reflectivity factor–rain rate at the ground (Z–R) relationships are obtained from simultaneous measurements by the three sensors. The Z–R relations derived from the three sensors show differences comparable to the ones seen in distinct climatic regions. In addition, the Z–R relationships are found to be sensitive to the data analysis method.

Rain properties controlling soil splash detachment

Laboratory experiments have been conducted to study the effects of various rain properties on sand detachment resulting from raindrop impact. Splash cups were exposed to simulated rainfall intensities ranging between 10 and 140 mm h−1. The detached sand was collected and weighed whereas rain intensity, equivalent drop diameter and fall velocity of raindrops were measured with an optical spectro-pluviometer (OSP). The properties of the simulated rain (i.e. median volume diameter and kinetic energy) were compared with those observed in natural conditions. Statistical analyses have been undertaken in order to evaluate which rain variable best predicts the mass of sand detached. Linear and non-linear correlations between the mass of detached sediment and the product of drop size (d) by drop velocity (v), i.e. DαVβ, with values of α varying between 1 and 6 and β between 0 and 3, have been computed. The results indicate that the coefficient of determination (R2) for α ranging between 3 and 5 and β lower or equal to 2 are satisfying. Although kinetic energy (D3V2) described splash detachment relatively well, the product of momentum by drop diameter (D4V) was slightly superior in describing splash detachment. Therefore, the momentum multiplied by the drop diameter is recommended as the best rain variable to describe splash detachment. Copyright © 2000 John Wiley & Sons, Ltd.

Measurement of simulated drop size distribution with an optical spectro pluviometer: sample size considerations

Knowledge of drop size distributions is important for deriving various rain erosivity parameters. This study investigates the potential of an optical spectro pluviometer (OSP) to measure drop size distributions. Particular attention is paid to the impact of drop sample size and derived erosivity parameters. An experimental set-up using a rainfall simulator and an OSP is described. The OSP allows a continuous real-time sampling of the drops. Results on drop size distributions and sampling effects are discussed. A simulation aimed at reproducing the sampling made with the widely used flour-pellet or filter-paper method is described. From this simulation, recommendations on the sample size of the collected drops needed for an accurate determination of median drop size and kinetic energy are given. Past studies reporting drop size characteristics have often used too small a sample for an adequate description of rain erosivity. Copyright © 1999 John Wiley & Sons, Ltd.

Measurement of simulated drop size distribution with an optical spectro pluviometer: sample size considerations

Measurement of simulated drop size distribution with an optical spectro pluviometer: sample size considerations

Performance of an optical spectro pluviometer in measuring basic rain erosivity characteristics

The Optical Spectro Pluviometer (OSP), an infrared optical device, is used to measure the rain drop size, fall velocity and intensity. The OSP, which works on the principle of measuring an optical shadow, enables independent measurement of drop size and drop velocity in real time and thus any parameter linked to rain erosivity. Simulated rain characteristics measured with the OSP are reported. Calibrations made in the laboratory using single raindrops allow an estimation of the accuracy of drop size and velocity measurements. The single drop calibration shows that drop diameters are measured with an accuracy that is at least 6% in the range 0.3–4.7 mm. Larger drops are detected but without quantification of their diameter. The accuracy in the drop fall velocity measurements varies from 1% for the lowest velocity (0.2 m s −1) to 20% for the highest velocity (10 m s −1). Simulated and natural raindrop-size distributions measured by the OSP are compared with distribution obtained by the filter paper method. Taking into account the uncertainties associated with the two methods, the raindrop-size distributions derived from the OSP and the filter paper method are in good agreement. To complete the calibration, rain intensity is derived from the raindrop-size distribution measurements and compared with values measured by a nearby rain gauge. For rain intensities less than 35 mm h −1 the OSP underestimates intensity by 12%. However, for rain intensities in excess of 35 mm h −1, the OSP underestimates intensity by 38%. The errors on the two widely used rain erosivity indices, the kinetic energy and the momentum, are estimated to be less than 4%.

The Optical Spectropluviometer Revisited

Abstract The optical spectropluviometer is a shadowgraph instrument able to measure independently the equivalent diameter and the fall speed of raindrops at ground level. Hardware and software modifications are proposed and tested. A modern digital signal processing system allows for the simultaneous sampling and analyzing of the signal delivered by the sensor. The IR light transmission is pulsed to avoid interference with natural radiation and the protection of the optics is improved. The validation procedure consists of comparing the rain rates derived from the measured drop size distributions with rain rates delivered by nearby rain gauges. The results obtained during 65 storm events show that the proposed improvements reduce the bias of the rain-rate estimation from 34% to 16%. Suggestions are given to further improve the performance of this instrument.

A New Optical Instrument for Simultaneous Measurement of Raindrop Diameter and Fall Speed Distributions

This paper presents a recently developed optical spectro-pluviometer. The principle of the device is based upon the optical occultation of an infrared light beam by falling drops. This allows the retrieval of raindrop-size and velocity distributions. The characteristics of the instrument are described in detail. Possible errors resulting either from instrumental limitations or from environment conditions of measurement are reviewed. Preliminary results regarding a 20 min rainshower observed near Paris in September 1980 are presented and discussed. The deduced total precipitation amount is in good agreement with other independent measurements. The results reveal small but significant differences between the deduced fall velocities and Gunn and Kinzer's observations. The relationship between rainfall bulk parameters (rainfall rate, water content, reflectivity factor and mean fall velocity) are obtained with small relative dispersion.