During droughts, soil evaporation is often constrained by water vapor transport through an air‐dry soil layer (DSL). Fick's water vapor diffusion is widely regarded as the only process for such ...transport; however, field studies conducted in arid and semi‐arid conditions showed measured evaporation rates higher than those predicted by diffusion. Therefore, transport processes other than diffusion could be relevant. To study the evaporation through a DSL, the same lysimeter column with 70 cm thick DSL as earlier applied in laboratory in Balugani et al. (2021), was installed in the field in Spain applying an original weighing setup to measure evaporation. The correlation between the measured evaporation and possible drivers of the water vapor transport were evaluated. With the DSL thickness of 70 cm in 2012 and 12 cm in 2015, the lysimeter recorded similar groundwater evaporation rates: 1.25 and 1.05 mm days−1, respectively; these rates were much larger than the laboratory recorded rates (0.3 mm days−1) and those estimated in this study using Hydrus1D accounting for non‐isothermal liquid water fluxes and water vapor diffusion. The main forcing driver of the field lysimeter evaporation was the soil profile temperature fluctuation, which concealed other less important forcing factors, that is, atmospheric pressure fluctuations and diffusion. A multivariate regression model to estimate evaporation was proposed, based on the profile temperature fluctuation, that, when added to the atmospheric pressure fluctuations, yielded reliable estimates of the cumulative evaporation measured in both 2012 and 2015.
Key Points
Development of an original lysimeter setup allowing for separate measurements of evaporation (E $E$) and groundwater evaporation (Eg ${E}_{g}$)
Under dry soil layer condition, lysimeter E=Eg $E={E}_{g}$, ∼1.25 mm days−1 in 2012 and 1.05 mm days−1 in 2015
Lysimeter evaporation correlated with profile soil temperature measurements and solar radiation changes; vapor diffusion was negligible
Modeling of water vapor transport through a dry soil layer (DSL), typically formed in the top soil during dry seasons in arid and semi‐arid areas, is still problematic. Previous laboratory ...experiments in controlled environments showed that the only vapor transport process through the DSL is by Fick's law of diffusion. However, field experiments exhibited consistently higher evaporation rates than predicted by diffusion flow only. Some proposed reasons for the mismatch were: (a) daily cycles of condensation and evaporation in the DSL due to changes in solar radiation; (b) wind effects on air movement in the DSL; (c) atmospheric pressure fluctuations; (d) nonlinear influence of the DSL thickness on the evaporation process. To link the laboratory experiments with field observations, we performed soil column experiments in the laboratory with thick (>50 cm) DSL, and with different wind speeds, two radiative lamp schedules (continuous and 12 h daily cycles) and different thicknesses of DSL. Atmospheric pressure, air temperature and humidity were measured continuously. The results show that the evaporation rates observed are larger than those predicted by diffusion flow only. We found that it was possible to model the evaporation rates as a function of atmospheric pressure fluctuations. In conclusion, atmospheric pressure fluctuations can induce evaporation rates in DSL larger than estimated by diffusion flow only, possibly explaining the discrepancy between laboratory and field evaporation rates.
Key Points
Evaporation with dry soil layer (DSL) investigated with laboratory experiments using soil columns under different evaporation conditions
Atmospheric pressure fluctuation has a large effect on the evaporation with thick DSL
Both daily changes in solar radiation and different DSL thickness had only a limited effect on soil evaporation with a thick DSL
Artificial lakes (reservoirs) are regulated water bodies with large stage fluctuations and different interactions with groundwater compared with natural lakes. A novel modelling study characterizing ...the dynamics of these interactions is presented for artificial Lake Turawa, Poland. The integrated surface-water/groundwater MODFLOW-NWT transient model, applying SFR7, UZF1 and LAK7 packages to account for variably-saturated flow and temporally variable lake area extent and volume, was calibrated throughout 5 years (1-year warm-up, 4-year simulation), applying daily lake stages, heads and discharges as control variables. The water budget results showed that, in contrast to natural lakes, the reservoir interactions with groundwater were primarily dependent on the balance between lake inflow and regulated outflow, while influences of precipitation and evapotranspiration played secondary roles. Also, the spatio-temporal lakebed-seepage pattern was different compared with natural lakes. The large and fast-changing stages had large influence on lakebed-seepage and water table depth and also influenced groundwater evapotranspiration and groundwater exfiltration, as their maxima coincided not with rainfall peaks but with highest stages. The mean lakebed-seepage ranged from ~0.6 mm day
−1
during lowest stages (lake-water gain) to ~1.0 mm day
−1
during highest stages (lake-water loss) with largest losses up to 4.6 mm day
−1
in the peripheral zone. The lakebed-seepage of this study was generally low because of low lakebed leakance (0.0007–0.0015 day
−1
) and prevailing upward regional groundwater flow moderating it. This study discloses the complexity of artificial lake interactions with groundwater, while the proposed front-line modelling methodology can be applied to any reservoir, and also to natural lake interactions with groundwater.
Field experiments have already proven that many tree species in water-limited environments (WLE) depend on groundwater. Typically, such trees survive dry seasons and droughts by uptake of water, ...directly from the groundwater body or from the capillary fringe, by rooting systems that may extend to several tens of meters depth. Such trees are also very efficient in finding soil moisture in the unsaturated zone, reducing groundwater recharge. Considering that WLE are typically characterized by low recharge, and that trees may use a significant amount of groundwater, this groundwater “consumption” should not be neglected in groundwater balancing, modeling and resources management. In practice, groundwater uptake by trees in WLE is either underestimated or disregarded because of limited knowledge about that phenomenon. This review discusses the current understanding of the hydrogeological role of trees in water-limited environments, the partitioning of tree transpiration into groundwater and unsaturated zone contributions and the integration of that partitioning in numerical groundwater models. Problems involved in this research are highlighted and possible future research directions are discussed.
•Integrated Hydrological Models (IHMs) are trustworthy but data-demanding.•Remote sensing (RS) and hydrogeophysics open exciting prospects for IHMs.•Combining RS and in-situ data for bias correction ...yields optimal climate forcings.•RS provides IHM input of surface/soil while hydrogeophysics, also of subsurface.•Evapotranspiration, soil moisture and stages are most used to constrain IHMs.
Integrated Hydrological Models (IHMs) dynamically couple surface and groundwater processes across the unsaturated zone domain. IHMs are data intensive and computationally demanding but can provide physically realistic output, particularly if sufficient input data of high quality is available. In-situ observations often have a small footprint and are time and cost-demanding. Satellite remote sensing observations, with their long time series archives and spatially semi-continuous gridded format, as well as hydrogeophysical observations with their flexible, ‘on-demand’ high-resolution data coverage, perfectly complement in-situ observations. We review the contribution of various satellite remote sensing products for IHM: (1) climate forcings, (2) parameters, (3) boundary conditions and (4) observations for constraining model calibration and data assimilation. Our review of hydrogeophysics focuses on the four mentioned IHM contributions, but we analyze them per data acquisition platform, i.e., surface, drone-borne and airborne hydrogeophysics. Finally, the review includes a discussion on the optimal use of satellite remote sensing and hydrogeophysical data in IHMs, as well as a vision for further improvements of data-driven, integrated hydrological modelling.
Sourcing subsurface evaporation (E
ss
) into groundwater (E
g
) and unsaturated zone (E
u
) components has received little scientific attention so far, despite its importance in water management and ...agriculture. We propose a novel sourcing framework, with its implementation in dedicated post-processing software called SOURCE (used along with the HYDRUS1D model), to study evaporation sourcing dynamics, define quantitatively "shallow" and "deep" water table conditions and test the applicability of water table fluctuation (WTF) and "bucket" methods for estimation of E
g
and E
u
separately.
For the "shallow" and "deep" water table we propose E
g
> 0.95E
ss
and E
g
= 0 criteria, respectively. Assessment of the WTF method allowed sourcing of very small fluxes otherwise neglected by standard hydrological methods. Sourcing with SOURCE software was more accurate than the standard "bucket" method mainly because of greater flexibility in spatio-temporal discretization. This study emphasized the dry condition relevance of groundwater evaporation which should be analysed by applying coupled flow of heat, vapour and liquid water.
Editor D. Koutsoyiannis; Associate editor S. Kanae
Spatio-temporal variability of recharge (
R) and groundwater evapotranspiration (
ET
g) fluxes in a granite Sardon catchment in Spain (∼80
km
2) have been assessed based on integration of various ...data sources and methods within the numerical groundwater MODFLOW model. The data sources and methods included: remote sensing solution of surface energy balance using satellite data, sap flow measurements, chloride mass balance, automated monitoring of climate, depth to groundwater table and river discharges, 1D reservoir modeling, GIS modeling, field cartography and aerial photo interpretation, slug and pumping tests, resistivity, electromagnetic and magnetic resonance soundings.
The presented study case provides not only detailed evaluation of the complexity of spatio-temporal variable fluxes, but also a complete and generic methodology of modern data acquisition and data integration in transient groundwater modeling for spatio-temporal groundwater balancing.
The calibrated numerical model showed spatially variable patterns of
R and
ET
g fluxes despite a uniform rainfall pattern. The seasonal variability of fluxes indicated: (1)
R in the range of 0.3–0.5
mm/d within ∼8 months of the wet season with exceptional peaks as high as 0.9
mm/d in January and February and no recharge in July and August; (2) a year round stable lateral groundwater outflow (
Q
g) in the range of 0.08–0.24
mm/d; (3)
ET
g=0.64, 0.80, 0.55
mm/d in the dry seasons of 1997, 1998, 1999, respectively, and <0.05
mm/d in wet seasons; (4) temporally variable aquifer storage, which gains water in wet seasons shortly after rain showers and looses water in dry seasons mainly due to groundwater evapotranspiration.
The dry season sap flow measurements of tree transpiration performed in the homogenous stands of
Quercus ilex and
Quercus pyrenaica indicated flux rates of 0.40 and 0.15
mm/d, respectively. The dry season tree transpiration for the entire catchment was ∼0.16
mm/d. The availability of dry season transpiration measurements considered as root groundwater uptake (
T
g), allowed estimation of dry season catchment groundwater evaporation (
E
g) as 0.48, 0.64, 0.39
mm/d for 1997, 1998 and 1999, respectively.
•The effects of a dry top soil layer (DSL) on evaporation depends on DSL thickness.•The thickness of the dry soil layer depends on the “dryness” of the system.•A DSL of ∼0.25 m greatly limits ...evaporation from bare soil with shallow water table.•In very dry conditions, DSL affects bare soil evaporation in semi-arid areas.
Bare soils and grasslands in arid and semi-arid conditions constitute a large portion of the earth surface. Evaporation, which is the main component of the water balance in these conditions, often takes place through a dry soil layer (DSL). There is no scientific agreement yet on the DSL effects on evaporation rates.
The implementations of three conceptual models of DSL-evaporation were tested for the simulation of evaporation rates in a semi-arid study area in Central Spain: (i) the daily-average model, based on the assumption that the daily average vapour transport in a DSL can be represented in analogy to isothermal liquid flow; (ii) the numerical model solving the Richards equation, in this case HYDRUS1D was used; and (iii) the pore-scale model, based on soil column experiments in laboratory conditions. The evaporation rates estimated by the three conceptual models for semi-arid field conditions were compared with the evaporation rates measured by an eddy covariance tower in the same area.
The results indicate that the daily-average conceptual model assumption, in which the DSL has no effects on evaporation, does not hold in very dry conditions. The numerical model solving the Richards equation was not able to simulate the effects of the DSL on evaporation rates. The evaporation estimates obtained by the pore-scale conceptual model were closest to the eddy covariance measurements during the dry season, however this model was applicable only to the relatively steady evaporation conditions during afternoons and only assuming spatially constant DSL thickness.
•We sourced groundwater soil evaporation including water vapor transport term.•Tree groundwater uptake was 50% of total T but only 6% of total ET.•Soil groundwater evaporation, usually neglected, was ...∼30% of total ET.
Studies on evapotranspiration partitioning under eddy covariance (EC) towers rarely address the separate effects of transpiration and evaporation on groundwater resources. Such partitioning is important to accurately assess groundwater resources, especially in arid and semi-arid areas.
The main objective of this study was to partition (evaluate separately) the evaporation and transpiration components of evapotranspiration, originated either from saturated or unsaturated zone, and estimate their contributions in a semi-arid area characterized by relatively shallow groundwater Table (0–10m deep).
Evapotranspiration, tree transpiration and subsurface evaporation were estimated with EC tower, using sap flow methods and HYDRUS1D model, respectively. To set up the HYDRUS1D model, soil material properties, soil moisture, soil temperature, soil matric potential and water table depth were measured in the area. The tree transpiration was sourced into groundwater and unsaturated zone components (∼0.017mmd−1 for both) and accounted for only ∼6% of the evapotranspiration measured by the EC tower (∼0.565mmd−1), due to the low canopy coverage in the study area (7%). The subsurface evaporation fluxes were also sourced into groundwater and unsaturated zone components using the SOURCE package, and their relative relevance in total evapotranspiration was assessed.
Subsurface evaporation was the main flux year-round (∼0.526mmd−1). During late autumn, winter and early spring time, the unsaturated zone evaporation was dominant, while in dry summer the relevance of groundwater evaporation increased, reaching one third of evapotranspiration, although errors in the water balance closure point still at its possible underestimation. The results show that, in arid and semi-arid areas with sparse vegetation, the often neglected groundwater evaporation is a relevant contribution to evapotranspiration, and that water vapor flow should be taken into account in the calculation of extinction depth.
Sap flow measurements conducted with thermal dissipation probes (TDPs) are vulnerable to natural temperature gradient (NTG) bias. Few studies, however, attempted to explain the dynamics underlying ...the NTG formation and its influence on the sensors' signal. This study focused on understanding how the TDP signals are affected by negative and positive temperature influences from NTG and tested the novel cyclic heat dissipation (CHD) method to filter out the NTG bias. A series of three experiments were performed in which gravity-driven water flow was enforced on freshly cut stem segments of Fagus sylvatica L., while an artificial temperature gradient (ATG) was induced. The first experiment sought to confirm the incidence of the ATG on sensors. The second experiment established the mis-estimations caused by the biasing effect of the ATG on standard TDP measurements. The third experiment tested the accuracy of the CHD method to account for the ATG biasing effect, as compared with other cyclic correction methods. During experiments, sap flow measured by TDP was assessed against gravimetric measurements. The results show that negative and positive ATGs were comparable in pattern but substantially larger than field NTGs. Second, the ATG bias caused an overestimation of the standard TDP sap flux density of ∼17 cm(3) cm(-2) h(-1) by 76%, and the sap flux density of ∼2 cm(3) cm(-2) h(-1) by over 800%. Finally, the proposed CHD method successfully reduced the max. ATG bias to 25% at ∼11 cm(3) cm(-2) h(-1) and to 40% at ∼1 cm(3) cm(-2) h(-1). We concluded that: (i) the TDP method is susceptible to NTG especially at low flows; (ii) the CHD method successfully corrected the TDP signal and resulted in generally more accurate sap flux density estimates (mean absolute percentage error ranging between 11 and 21%) than standard constant power TDP method and other cyclic power methods; and (iii) the ATG enforcing system is a suitable way of re-creating NTG for future tests.
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