Lester P. Simmonds

Use of microlysimeters to measure evaporation from sandy soils

Evaporation from soil can be a major component of crop water balance and land surface energy balance. A number of different applications of the microlysimeter method to measure evaporation from soil have been used in recent studies. Microlysimeters were used extensively in three sandy soils for this study. Measurement of evaporation from microlysimeters with different dimensions and of different ages allows discussion of the sources of error inherent in the method. The evaporation recorded from microlysimeters of diameters 214 mm, 152 mm and 51 mm was not significantly different. A comparison of 100 mm and 200 mm deep microlysimeters showed that depth had no significant influence during the first 2 days after extraction from the soil profile. For periods beginning 2 or more days after rain, significant differences in evaporation owing to depth may not occur for up to 7 days. Soil cores extracted at different times showed significant differences in evaporation immediately following a rain event, and no significant differences 2 or more days thereafter. This period of significant difference was extended to about 4 days when the method was used within a crop (i.e. root extraction of water in the field significant). A protocol for use of microlysimeters is developed from these results.

Measurement of Evaporation from Bare Soil and its Estimation Using Surface Resistance

Evaporation from soil, Es, is important to land surface energy balance and has been estimated in many studies using a surface resistance approach. We investigate the accuracy of this approach using detailed measurement and simulation. Hourly evaporation rates were measured using microlysimeters and load cells at two semiarid sites with bare soil. A numerical model of water (liquid and vapor) and heat fluxes in a soil profile (the soil water, energy, and transpiration (SWEAT) model) provided an accurate simulation of measured evaporation rates. Using output from SWEAT, relationships between soil resistance rs and soil surface water content θs (0–20 and 0–50 mm) are determined and are then used to estimate Es. These rs-based models performed well over a period of several days but provided poor estimates of Es on an hourly or even a daily basis. A characteristic divergence between measured Es rates and potential evaporation rates at a time during the early daylight hours was not well simulated by rs-based models. Anrs(θs) function for a similar soil at a different location underestimated Es by about 60%, Our work suggests that rs calculated from both evaporative demand and near-surface soil water content θs is likely to be more accurate.

Calibrating a soil water and energy budget model with remotely sensed data to obtain quantitative information about the soil

A soil water energy and transpiration model (SWEAT) coupled with a microwave emission model (MICRO-SWEAT) was used to predict the microwave brightness temperature of both bare and corn plots during a drying cycle. The predicted microwave brightness temperatures compared favorably to measurements made with an L band (21 cm, 1.4 GHz) passive microwave radiometer. In addition, SWEAT successfully modeled time series of soil water content and soil temperature. The modeled brightness temperature for the bare soil was most sensitive to the parameters describing the soil water retention and conductivity characteristics. These were predicted by varying each parameter in turn until there was a minimum between the measured and modeled brightness temperature. The predicted parameters were in agreement with the measured values to within the experimental error. The microwave brightness temperatures estimated for the corn soil were sensitive to the vegetation parameters as well as to the soil hydraulic properties.

Including the heat storage term in sap flow measurements with the stem heat balance method

The importance of the change in stem temperature and therefore the heat storage term in the stem heat balance method of measuring sap flow is examined. Results from a range of measurements on a model stem, potted sunflower plants in a glasshouse, and Guiera senegalensis shrubs in the Sahel, Niger, are presented. A novel analysis of the heat balance in zero flow conditions allows the accurate determination of the gauge radial conductance and the stem segment heat capacity, both of which are required for accurate sap flow measurement with good dynamic resolution in low flow conditions. In high sap flow conditions the change in heat storage constitutes only a small component of the balance, and can be neglected, especially for small stems. The improved accuracy and dynamic resolution for stems of any size if heat storage is included allowed the measurement of low night-time flows during rehydration, and of redistribution of water between stems of G. senegalensis bushes in the field following rain.

Estimation of transpiration by single trees: comparison of sap flow measurements with a combination equation

Abstract Sap flow estimates for whole trees (scaled from measurements on selected branches using the heat balance method) were compared with estimates of transpiration based on porometry in a study of poplar trees in an agroforestry system in the south of the UK. Sap flow showed good agreement with the transpiration rate estimated using the Penman-Monteith equation with measured stomatal conductance (R2 = 0.886) on six selected days during the season. The dominant environmental variable influencing transpiration was the vapour pressure deficit, as the “aerodynamic term” in the Penman-Monteith equation accounted for more than 70% of daily total transpiration, with the rest due to the “radiation component”. Stomatal conductance, estimated by inverting the Penman-Monteith equation from continuous measurements of sap flow over 55 days, was used to determine the parameters for a multiplicative stomatal conductance model. For an independent data set there was better agreement between measured sap flow and transpiration predicted from the stomatal conductance (R2 = 0.90) than for calculated and predicted stomatal conductance (R2 = 0.51).

Using a modeling approach to predict soil hydraulic properties from passive microwave measurements

A soil water and energy budget model coupled with a microwave emission model (MICRO-SWEAT) was used to predict the diurnal courses of soil surface water content and microwave brightness temperatures during a number of drying cycles on soils of contrasting texture that were either cropped or bare. The parameters describing the soil water retention and conductivity characteristics [saturated hydraulic conductivity, air entry potential, bulk density, and the exponent (b) describing the slope of the water release curve] had a strong influence on the modeled bare-soil microwave brightness temperatures. These parameters were varied until the error between the remotely sensed and modeled brightness temperatures was minimized, leading to their predicted values. These predictions agreed with the measured values to within the experimental error. The modeled brightness temperature for a soybean-covered soil was sensitive to some of the vegetation parameters (particularly to the optical depth), in addition to the soil hydraulic properties. Preliminary findings suggest that, given an independent estimate of the vegetation parameters, it may still be possible to estimate the soil hydraulic properties under a moderate vegetation canopy.

Measurement of surface redistribution of rainfall and modelling its effect on water balance calculations for a millet field on sandy soil in Niger

Abstract During rain there can be substantial redistribution of water at the surface of sandy soils in the Sudano-Sahelian zone, because of localised runoff and runon. This results in variable infiltration over a field. Measurements of spatial variability in infiltration and crop growth were made in a millet field at the southern supersite of the HAPEX-Sahel experiment in Niger. Infiltration was calculated from the change in soil water storage measured using a neutron probe at up to 33 locations, before and after rain storms exceeding 10 mm. Data were obtained for five storms in 1993 and 1994. Infiltration varied from 0.3 to 3.4 times the recorded rainfall, though more than 80% of the locations had infiltration between 0.6 and 1.2 times the recorded rainfall. There was some consistency between storms, with locations at the extremes of infiltration having consistently high or low infiltration. The amount of infiltration had little discernable influence on crop growth, other than possibly at the very dry and very wet sites, where growth was reduced. The soil water balance model, SWIM, was used to assess the consequences of variable infiltration and crop growth on the partitioning of water losses between evaporation and drainage in 1992. Simulation of variable infiltration suggested that it has relatively little effect on evaporation, but considerable effect on point drainage. Once there was sufficient infiltration to cause drainage (which was achieved in all but extreme runoff areas), there was a linear relationship between any further cumulative infiltration and the annual loss through drainage, with typically more than 70% of any further input being lost as drainage. This linear relationship meant that on a field scale, variable infiltration had minimal effect on drainage, with increased drainage from runon areas tending to be at the expense of reduced drainage from runoff areas.

The impact of sparse millet crops on evaporation from soil in semi-arid Niger

Abstract Direct evaporation from soil is an important component of crop water balances in semi-arid environments. The effects of a crop and of crop management on this water loss from the soil have been estimated in the past using combinations of field measurement and simple models, but there are inconsistencies in the conclusions reached. This paper presents data from water balance studies on millet crops in Niger during the 1991 and 1993 seasons. Evaporation from soil ( E s ) was measured under two contrasting cropping intensities in both years using the microlysimeter method. small seasonal reductions in E s from the higher intensity crop were recorded (12% and 16% in 1991 and 1993, respectively). significant reductions in daily E s were: (1) nearly all recorded within a limited period in the season when there were large differences in transpiring leaf area; (2) recorded for both high ( > =2 mm day −1 ) and low ( −1 ) values of E s . These data indicate that soil drying by root water uptake contributed to the reduction of E s . Increased shading of the soil by the crop canopy does not result in a proportional reduction of E s . Two simple models for estimating ES beneath crops (Ritchie, 1972; Cooper et al., 1983) are compared with field data and an improvement to the Ritchie model is suggested. Two new parameters are introduced to estimate the relative importance of (1) the atmospheric vapour pressure deficit to potential evaporation and (2) root water uptake to soil drying. The brief description of environment and crop included in the new approach allows identification of the environments in which there is scope for substantial reduction in E s through crop management.

Analysis of maize–common bean intercrops in semi-arid Kenya

Maize ( Zea mays L.) and common bean ( Phaseolus vulgaris L.) were each sown at four plant densities, including zero, in a bivariate factorial design at Kiboko Rangeland Research Station, Kenya during the long and short rains of 1990. The design gave nine intercrops with different proportions of maize and beans, and six sole crops, three of maize and three of beans. Seed yields in both the sole crops were not significantly affected by plant density, so the mean yield was used to calculate the Land Equivalent Ratio (LER), which averaged 1·09 in the long rains but only 0·87 in the short rains. These low values were apparently due to the fact that beans failed to nodulate and fix nitrogen in the study area. The difference in LER between seasons was probably caused by differences in the amount and distribution of rain in relation to crop growth. Maize was more competitive than bean, each maize plant being equivalent to between 0·7 and 3·4 bean plants depending upon the treatment and the season.

Transpiration efficiencies of maize and beans in semi-arid Kenya

Maize and bean were grown under varying levels of nitrogen fertilizer, plant population, and irrigation at Kiboko, Kenya in the short rains 1990, 1991, 1992 and the long rains 1991. The production of dry matter was not affected significantly by any treatment, because treatments only had a small impact on the balance between evaporation and transpiration. In all seasons the greatest loss of water from the profile was through direct evaporation from the soil surface. Transpiration was always less than 25% of rainfall. The ratio of transpiration (T) to evapotranspiration (E + T) was small (0.23), but increased from 0.15 to 0.40 as rainfall increased from 158 mm in the long rains 1991 to 470 mm in the short rains 1992. Treatments had little impact on the balance between transpiration and evaporation from the soil surface. The average transpiration efficiencies for maize and bean were 89 and 29 kg shoot dry matter ha−1 mm−1, respectively. For each crop there was a 60% change in transpiration efficiency between the short and the long rain season which could be accounted for by differences in saturation vapour pressure deficit.