Potential Evaporation Data Research Articles

Revisiting the Thornthwaite Mather procedure for baseflow and groundwater storage predictions in sloping and mountainous regions

Hillslope aquifers regulate streamflow and are a critical potable and irrigation water source, especially in developing countries. Knowing recharge and baseflow is essential for managing these aquifers. Methods using available data to calculate recharge and baseflow from aquifers are not valid for uplands. This paper adapts the Thornthwaite and Mather (T-M) procedure from plains to sloping and mountainous regions by replacing the linear reservoir with a zero-order aquifer. The revised T-M procedure was tested over four years in two contrasting watersheds in the humid Ethiopian highlands: the 57 km2 Dangishta with a perennial stream and the nine km2 Robit Bata, where the flow ceased four months after the end of the rain phase. The monthly average groundwater tables were predicted with an accuracy ranging from satisfactory to good for both watersheds. Baseflow predictions were “very good” after considering the evaporation from shallow groundwater in the valley bottom during the dry phase in Dangishta. We conclude that the T-M procedure is ideally suited for calculating recharge, baseflow and groundwater storage in upland regions with sparse hydrological data since the procedure uses as input only rainfall and potential evaporation data that are readily available together with an estimate of the aquifer travel time.

Soil surface-atmosphere interaction in a monitored embankment constructed with two compacted lime-treated soils

This study was carried out in an instrumented and monitored embankment divided into two symetrical sections and constructed with compacted treated soils (i.e., a silty soil and a clayey soil) exposed to the same atmospheric conditions in a continental climate, with oceanic influences. It aimed to investigate changes of bare soil surface moisture (w) and corresponding suction (s) as result of soil water evaporation processes on a monthly time scale. Seasonal variations in the w (and s) measurement in both soil surfaces show overall consistency with the meteorological measurement within the study area. The paper also examines the ability of four air temperature-based potential evaporation (PET) formulations to capture the process of evaporation at the site. Results indicated that soil water evaporation is controlled by both atmospheric and soil conditions. And, during the most significant drying time period, the measured s consistently increased and the corresponding w decreased suggesting a relatively significant water evaporation effect. However, the monthly predicted PET data varied from a maximum of over 120 mm/month to less than 50 mm/month during the drying time, depending on the used method. The continuously monitored soil surface suctions are used for discussing the variations of evaporation according to the predicted PET method and time period at the site.

Gravimetry-based water storage shifting over the China-India border area controlled by regional climate variability

The regional water storage shifting causes nonstationary spatial distribution of droughts and flooding, leading to water management challenges, environmental degradation and economic losses. The regional water storage shifting is becoming evident due to the increasing climate variability. However, the previous studies for climate drivers behind the water storage shifting are not rigorously quantified. In this study, the terrestrial water storage (TWS) spatial shifting pattern during 2002–2017 over the China-India border area (CIBA) is developed using the Gravity Recovery and Climate Experiment (GRACE), suggesting that the Indus-Ganges-Brahmaputra basin (IGBB) was wetting while the central Qinghai-Tibet Plateau (QTP) was drying. Similar drying and wetting patterns were also found in the precipitation, snow depth, Palmer Drought Severity Index (PDSI) and potential evaporation data. Based on our newly proposed Indian monsoon (IM) and western North Pacific monsoon (WNPM) variation indices, the water shifting pattern over the CIBA was found to be affected by the weakening of the variation of IM and WNPM through modulating the regional atmospheric circulation. The weakening of IM and WNPM variations has shown to be attributed to the decreasing temperature gradient between the CIBA and the Indian Ocean, and possibly related to increasing regional temperatures associated with the increasing global temperature. As the global warming intensifies, it is expected that the regional TWS shifting pattern over the CIBA will be further exaggerated, stressing the need of advancing water resources management for local communities in the region.

Assessment of lake surface dynamics using satellite imagery and in-situ data; case of Lake Ngami in North-West Botswana

The understanding of surface dynamics of Lake Ngami, a part of Okavango river system is hindered by paucity of data. The aim of this study was to assess the lake surface dynamics using satellite imagery and in-situ data.Surface dynamics of Lake Ngami were monitored using remote sensing methods to complement the available inadequate in-situ measurements. Landsat imagery of the year 1979 to year 2016 were used. The Lake boundary was mapped using the Modified Normalised Difference Water Index (MNDWI), where use was made of Histogram thresholding segmentation method in extracting the water pixels. Remotely sensed actual evapotranspiration (ETa) was estimated using Simplified Surface Energy Balance (SSEB) Modelling Approach; while the satellite derived rainfall estimates were obtained from Tropical Rainfall Measuring Mission (TRMM) 3B43V7 satellite imagery product. Precipitation and potential evaporation data from meteorological stations were used to correct the bias of the satellite products. The SSEB method involved the estimation of evaporation fraction using the MODIS Land Surface Temperature and Emissivity 8-day product (MOD11A2) and GLDAS 0.25° Potential Evapotranspiration version 2.1 for the estimation of potential evapotranspiration.The lake extraction process displayed good results with the average Kappa coefficient and overall accuracy of 0.93 and 96.6%, respectively. The maximum extent of Lake Ngami was observed in the year 2012 at 272 km2. The ETa was found to be in range of 700 mm/year in dry regions to about 2000 mm/year in Okavango delta swamps. From the study, it was observed that after the 2009 flood-event, the precipitation occurring during the rainy season (October–March) at Angola Highlands had a positive relationship with the Lake Ngami surface area, with Pearson correlation of 0.70. Similar findings were noticed when the lake surface area trend was observed against the in-situ flow data at the Mohembo hydrometric station, which is the inflow of the Okavango delta. This suggests that since the floods of 2009, the precipitation in the Angola Highlands has a major influence in the surface dynamics of Lake Ngami.

Exploring the relationships between warm-season precipitation, potential evaporation, and “apparent” potential evaporation at site scale

Abstract. Bouchet's complementary relationship and the Budyko hypothesis are two classic frameworks that are inter-connected. To systematically investigate the connections between the two frameworks, we analyze precipitation, pan evaporation, and potential evaporation data at 259 weather stations across the United States. The precipitation and pan evaporation data are from field measurement and the potential evaporation data are collected from a remote-sensing dataset. We use pan evaporation to represent “apparent” potential evaporation, which is different from potential evaporation. With these data, we study the correlations between precipitation and potential evaporation, and between precipitation and “apparent” potential evaporation. The results show that 93 % of the study's weather stations exhibit a negative correlation between precipitation and “apparent” potential evaporation. Also, the aggregated data cloud of precipitation vs. “apparent” potential evaporation with 5312 warm-season data points from 259 weather stations shows a negative trend in which “apparent” potential evaporation decreases with increasing precipitation. On the other hand, no significant correlation is found in the data cloud of precipitation vs. potential evaporation, indicating that precipitation and potential evaporation are independent. We combine a Budyko-type expression, the Turc–Pike equation, with Bouchet's complementary relationship to derive upper and lower Bouchet–Budyko curves, which display a complementary relationship between “apparent” potential evaporation and actual evaporation. The observed warm-season data follow the trend of the Bouchet–Budyko curves. Our study shows the consistency between Budyko's framework and Bouchet's complementary relationship, with the distinction between potential evaporation and “apparent” potential evaporation. The formulated complementary relationship can be used in quantitative modeling practices.

Machine learning assessments of soil drying for agricultural planning

The hydrologic processes of wetting and drying play a crucial role in agricultural activities involving heavy equipment on unpaved terrain. When soil conditions moisten, equipment can become mired, causing expensive delays. While experienced users may assess soil conditions before entering off-road areas, novice users or those who must remotely assess sites before traveling may have difficulty assessing conditions reliably. One means of assessing dryness is remotely-monitored in situ sensors. Unfortunately, land owners hesitate to place sensors due to monetary costs, complexity, and sometimes infeasibility of physical visits to remote locations. This work addresses these limitations by modeling the wetting/drying process through machine learning algorithms fed by hydrologic data – remotely assessing soil conditions using only publicly-accessible information. Classification trees, k-nearest-neighbors, and boosted perceptrons deliver statistical soil dryness estimates at a site located in Urbana, IL. The k-nearest-neighbor and boosted perceptron algorithms both performed with 91–94% accuracy, with most misclassifications falling within calculated margins of error. These analyses demonstrate that reasonably accurate predictions of current soil conditions are possible with only precipitation and potential evaporation data. These two values are measured throughout the continental United States and are likely to be available globally from satellite sensors in the near future. Through this type of approach, agricultural management decisions can be enabled remotely, without the time and expense of on-site visitations or extensive ground-based sensory grids.

Disinformative data in large-scale hydrological modelling

Abstract. Large-scale hydrological modelling has become an important tool for the study of global and regional water resources, climate impacts, and water-resources management. However, modelling efforts over large spatial domains are fraught with problems of data scarcity, uncertainties and inconsistencies between model forcing and evaluation data. Model-independent methods to screen and analyse data for such problems are needed. This study aimed at identifying data inconsistencies in global datasets using a pre-modelling analysis, inconsistencies that can be disinformative for subsequent modelling. The consistency between (i) basin areas for different hydrographic datasets, and (ii) between climate data (precipitation and potential evaporation) and discharge data, was examined in terms of how well basin areas were represented in the flow networks and the possibility of water-balance closure. It was found that (i) most basins could be well represented in both gridded basin delineations and polygon-based ones, but some basins exhibited large area discrepancies between flow-network datasets and archived basin areas, (ii) basins exhibiting too-high runoff coefficients were abundant in areas where precipitation data were likely affected by snow undercatch, and (iii) the occurrence of basins exhibiting losses exceeding the potential-evaporation limit was strongly dependent on the potential-evaporation data, both in terms of numbers and geographical distribution. Some inconsistencies may be resolved by considering sub-grid variability in climate data, surface-dependent potential-evaporation estimates, etc., but further studies are needed to determine the reasons for the inconsistencies found. Our results emphasise the need for pre-modelling data analysis to identify dataset inconsistencies as an important first step in any large-scale study. Applying data-screening methods before modelling should also increase our chances to draw robust conclusions from subsequent model simulations.

Predicting discharge and sediment for the Abay (Blue Nile) with a simple model

AbstractModels accurately representing the underlying hydrological processes and sediment dynamics in the Nile Basin are necessary for optimum use of water resources. Previous research in the Abay (Blue Nile) has indicated that direct runoff is generated either from saturated areas at the lower portions of the hillslopes or from areas of exposed bedrock. Thus, models that are based on infiltration excess processes are not appropriate. Furthermore, many of these same models are developed for temperate climates and might not be suitable for monsoonal climates with distinct dry periods in the Nile Basin. The objective of this study is to develop simple hydrology and erosion models using saturation excess runoff principles and interflow processes appropriate for a monsoonal climate and a mountainous landscape. We developed a hydrology model using a water balance approach by dividing the landscape into variable saturated areas, exposed rock and hillslopes. Water balance models have been shown to simulate river flows well at intervals of 5 days or longer when the main runoff mechanism is saturation excess. The hydrology model was developed and coupled with an erosion model using available precipitation and potential evaporation data and a minimum of calibration parameters. This model was applied to the Blue Nile. The model predicts direct runoff from saturated areas and impermeable areas (such as bedrock outcrops) and subsurface flow from the remainder of the hillslopes. The ratio of direct runoff to total flow is used to predict the sediment concentration by assuming that only the direct runoff is responsible for the sediment load in the stream. There is reasonable agreement between the model predictions and the 10‐day observed discharge and sediment concentration at the gauging station on Blue Nile upstream of Rosaries Dam at the Ethiopia–Sudan border. Copyright © 2009 John Wiley & Sons, Ltd.

Transient simulations of flow and transport in the Chalk unsaturated zone

Chalk is a fractured porous medium composed of matrix blocks bounded by interconnected fractures. Furthermore, there are a number of commonly observed, and apparently contradictory, phenomena. These include: a fast water table response, slow solute migration and very little solute dispersion. To the authors' knowledge, no unified physically-based, numerical model has been able to reconcile all of these points. In this paper, these issues were addressed through the development of a transient dual-permeability model of flow and transport. The behaviour of the model was explored through a hypothetical scenario. The one-dimensional model featured a fixed water table at 10 m depth. A recharge time-series, derived using the Penman-Grindley model in conjunction with daily precipitation and monthly potential evaporation data from the Kennet valley, Berkshire, was applied at the top of the model. A conservative solute (that could be tritium, chloride or nitrate) was applied to the recharge for 1 year. Subsequent years involved the input of clean water. The simulations suggested that although transient effects have been previously neglected, overlying soil and gravelly chalk layers are likely to cause significant attenuating effects on flow such that most flow occurs through the matrix. Fracture flow was found to be episodic and infrequent and represented between 17% and 30% of the annual recharge.

Application of linear and nonlinear techniques in river flow forecasting in the Kilombero River basin, Tanzania / Application de techniques linéaires et non-linéaires à la prévision des débits dans le bassin de la Rivière Kilombero, en Tanzanie

Modelling of the rainfall–runoff transformation process and routing of river flows in the Kilombero River basin and its five sub-catchments within the Rufiji River basin in Tanzania was undertaken using three system (black-box) models—a simple linear model, a linear perturbation model and a linear varying gain factor model—in their linear transfer function forms. A lumped conceptual model—the soil moisture accounting and routing model—was also applied to the sub-catchments and the basin. The HEC-HMS model, which is a distributed model, was applied only to the entire Kilombero River basin. River discharge, rainfall and potential evaporation data were used as inputs to the appropriate models and it was observed that sometimes the system models performed better than complex hydrological models, especially in large catchments, illustrating the usefulness of using simple black-box models in datascarce situations.

RCM rainfall for UK flood frequency estimation. I. Method and validation

Regional Climate Models (RCMs) are a significant improvement over Global Climate Models (GCMs) in terms of their representation of rainfall at the spatial and temporal scale required for flood modelling. This paper presents the use of catchment rainfall and potential evaporation data derived directly from RCM data, as input to a spatially generalised continuous simulation rainfall-runoff model, for 15 catchments across Great Britain. The RCM (HadRM3H) is based on a ∼25 km grid across Europe and, rather than being nested within a GCM, is driven with boundary conditions determined by 15 years' of data from the ECMWF's global model for the period 1979–1993 (ERA-15). As the latter model continually assimilates observations of surface and atmospheric weather elements, output from the RCM can be expected to match observed conditions to a reasonable extent, enabling a direct comparison with simulations using observed input data. The rainfall-runoff model simulates continuous flow time-series, from which flood frequency curves are derived using a standard peaks-over-threshold method. The study covers return periods up to about 10 years (due to limited data availability) and relatively small catchments (<500 km 2). Overall, flood frequency curves derived using RCM inputs compare well with those using observed rainfall and evaporation inputs, although with a slight tendency to underestimate. Only two catchments show particularly bad underestimation however, which is due to their input RCM rainfall being significantly lower than that observed. Flood frequency estimation errors are in fact closely related to errors in annual average rainfall amounts for the RCM, which future improvements to the RCM will hopefully overcome.

EMPIRICAL FACTORS FOR ESTIMATING OPEN‐WATER EVAPORATION FROM POTENTIAL EVAPORATION

ABSTRACTFor many years, the standard method for estimating rates of evaporation from open water has been to apply empirical factors to potential evaporation data. However, significant errors can occur through the use of these factors, essentially due to the limitations of the measurements upon which they are based. New empirical factors are presented here for use with the Meteorological Office Rainfall and Evaporation Calculation System for grass potential evaporation and Penman potential evaporation. However, uncertainty in the estimated evaporation rates remains at least 5–12 mm per month.

The effects of large-scale afforestation and climate change on water allocation in the Macquarie River catchment, NSW, Australia

Widespread afforestation has been proposed as one means of addressing the increasing dryland and stream salinity problem in Australia. However, modelling results presented here suggest that large-scale tree planting will substantially reduce river flows and impose costs on downstream water users if planted in areas of high runoff yield. Streamflow reductions in the Macquarie River, NSW, Australia are estimated for a number of tree planting scenarios and global warming forecasts. The modelling framework includes the Sacramento rainfall-runoff model and IQQM, a streamflow routing tool, as well as various global climate model outputs from which daily rainfall and potential evaporation data files have been generated in OzClim, a climate scenario generator. For a 10% increase in tree cover in the headwaters of the Macquarie, we estimate a 17% reduction in inflows to Burrendong Dam. The drying trend for a mid-range scenario of regional rainfall and potential evaporation caused by a global warming of 0·5°C may cause an additional 5% reduction in 2030. These flow reductions will decrease the frequency of bird-breeding events in Macquarie Marshes (a RAMSAR protected wetland) and reduce the security of supply to irrigation areas downstream. Inter-decadal climate variability is predicted to have a very significant influence on catchment hydrologic behaviour. A further 20% reduction in flows from the long-term historical mean is possible, should we move into an extended period of below average rainfall years, such as occurred in eastern Australia between 1890 and 1948. Because current consumptive water use is largely adapted to the wetter conditions of post 1949, a return to prolonged dry periods would cause significant environmental stress given the agricultural and domestic water developments that have been instituted.

The effects of large-scale afforestation and climate change on water allocation in the Macquarie River catchment, NSW, Australia

Widespread afforestation has been proposed as one means of addressing the increasing dryland and stream salinity problem in Australia. However, modelling results presented here suggest that large-scale tree planting will substantially reduce river flows and impose costs on downstream water users if planted in areas of high runoff yield. Streamflow reductions in the Macquarie River, NSW, Australia are estimated for a number of tree planting scenarios and global warming forecasts. The modelling framework includes the Sacramento rainfall-runoff model and IQQM, a streamflow routing tool, as well as various global climate model outputs from which daily rainfall and potential evaporation data files have been generated in OzClim, a climate scenario generator. For a 10% increase in tree cover in the headwaters of the Macquarie, we estimate a 17% reduction in inflows to Burrendong Dam. The drying trend for a mid-range scenario of regional rainfall and potential evaporation caused by a global warming of 0·5°C may cause an additional 5% reduction in 2030. These flow reductions will decrease the frequency of bird-breeding events in Macquarie Marshes (a RAMSAR protected wetland) and reduce the security of supply to irrigation areas downstream. Inter-decadal climate variability is predicted to have a very significant influence on catchment hydrologic behaviour. A further 20% reduction in flows from the long-term historical mean is possible, should we move into an extended period of below average rainfall years, such as occurred in eastern Australia between 1890 and 1948. Because current consumptive water use is largely adapted to the wetter conditions of post 1949, a return to prolonged dry periods would cause significant environmental stress given the agricultural and domestic water developments that have been instituted.

The Plynlimon water balance 1969-1995: the impact of forest and moorland vegetation on evaporation and streamflow in upland catchments

Abstract. The Plynlimon experiment in mid-Wales, designed to determine the extent to which coniferous plantation increases evaporation losses and reduces streamflow relative to upland grassland, has now been yielding data since 1969 from the grassland Wye and the 67% forested Severn catchments. Water balance analyses of the early data indicated significantly higher evaporation rates from the forested catchment and studies of the hydrological processes involved attributed this to the high loss rates of precipitation intercepted by the forest canopy. Models based on these process studies predicted losses from the forested catchment that were similar but marginally higher than those determined by the catchment water balance. As the data sets from the catchments increased in length and a detailed reassessment of the ratings of the streamflow gauging structures was completed the updated water balances continued to show a significantly greater evaporation loss from the forested catchment, but the gap between the forest water balance and the model predictions widened. Furthermore Hudson and Gilman (1993), using the best data sets then available, identified downward trends in the evaporation from both catchments which the models did not reproduce and for which no obvious physical or physiological explanation was forthcoming. This dictated a major reassessment of the longer data sets, using the more powerful data processing techniques now available, to identify and eliminate any errors and inconsistencies. This paper describes the reassessment of the precipitation data and the estimates of potential evaporation and presents the water balance results emerging from the revised data sets. The revised results indicate that the evaporation losses from the grassland Wye catchment remained broadly similar to the potential evaporation estimates throughout the 1969-1995 period. The losses from the forested area of the Severn catchment declined from a level some 61% above that of the grassland in 1972 to a level only 18% higher before the start of felling in 1985. This downward trend continued as the felling and re-planting progressed. Over the period since 1990 the forest catchment losses appear to have stabilised at some 5-10% below those of the grassland catchment. Using the revised precipitation and potential evaporation data, process based models over-predict the forest catchment evaporation throughout the period and do not mirror the re-felling decline. Possible reasons for this apparent decline in evaporation rates are discussed.

The effects of climate changes on aquifer storage and river baseflow

Abstract The effects of changes in climate on aquifer storage and groundwater flow to rivers have been investigated using an idealized representation of the aquifer/river system. The generalized aquifer/river model can incorporate spatial variability in aquifer transmissivity and is applied with parameters characteristic of Chalk and Triassic sandstone aquifers in the United Kingdom, and is also applicable to other aquifers elsewhere. The model is run using historical time series of recharge, estimated from observed rainfall and potential evaporation data, and with climate inputs perturbed according to a number of climate change scenarios. Simulations of baseflow suggest large proportional reductions at low flows from Chalk under high evaporation change scenarios. Simulated baseflow from the slower responding Triassic sandstone aquifer shows more uniform and less severe reductions. The change in hydrological regime is less extreme for the low evaporation change scenario, but remains significant for the Chalk aquifer.