Increasing pressure on groundwater due to land use change (
The relationship of vegetation cover and groundwater resources has drawn considerable scientific attention over the last decades. Many studies have shown that deforestation by logging or of natural origin (forest fire, wind damage) increased the average runoff from the affected area (
Comparative water balance studies of forest and low vegetation covers have generally shown higher water use of forest cover (
Due to climate change, air temperature is expected to rise significantly during this century (
The impact of climate change on groundwater resources was reviewed lately by
In light of the water balance uncertainties and the increasing pressures on groundwater resources due to future climate change, a comparative water balance study of an oak forest and fallow vegetation plots was initiated in a drought-threatened lowland environment in north-eastern Hungary. Water balance components were estimated by the Hydrus 1-D numerical model (
Our research questions were the following: (1) what is the magnitude of evapotranspiration components in the two different land-uses; and (2) how has groundwater consumption evolved during dry and wet growing seasons?
The study plots are situated in the north-east part of Hungary at latitude 47° 58’ N and longitude 21° 42’ E (
The plots were located on the discharge area of the local phreatic groundwater flow system with shallow groundwater levels and groundwater supplies from the adjacent areas. Both study plots were selected so that the elevation of the plots above the nearby ditch was almost the same. The generally similar site conditions of the field plots made it possible to compare water balance components and groundwater consumption. The approximately 300 m distance from the nearby ditch minimized any effect that floods had on water table levels. Surface runoff was not observed since the slope of the plots was less than 2‰ (
The naturally regenerated oak forest had 60% pedunculate oak (
The vertical distribution of the root system was surveyed
Soil analyzes included the sieving and hydrometer analyzes of particle size distribution of the soil samples, taken at 0.2 m intervals down to a depth of three meters. At both plots the soil texture was compacted fine sand (0.02-0.2 mm) close to the surface varying between 80-99%. The clay and silt fractions were high below one meter depth at both plots and reach 30-35% and 20-25%, respectively.
Three repetition of undisturbed soil samples for water retention were analyzed at depths of 0.1, 0.3, 0.5, 0.7, 0.9 and 1.2 m by cylinders of 100 cm3 (
The maximum of the Leaf Area Index (LAI - m2 m-2) was estimated by collecting leaf litter on the ground. At the oak plot, newly defoliated leaves were collected carefully from five locations (1 x 1 m) during late autumn of 2007 and dried in an oven (105 °C for 24 hours). Determination of LAI included the calculation of the ratio of weight to leaf area for a subset of leaves and then for the whole sample. The average LAI of the samples was 3.9 m2 m-2. The 16-day Enhanced Vegetation Index (EVI) product of MODIS (Moderate Resolution Imaging Spectroradiometer) was used to describe the seasonal change of LAI. The 250 m resolution EVI was converted to LAI using the relationship proposed by
At the fallow plot, the maximum leaf area index was estimated by leaf collection from three locations (0.5 x 0.5 m). All the collected leaves were scanned and the leaf area was determined by grid counting. The mean LAI of the three samples was 1.1 m2 m-2. The same leaf area index was used throughout the vegetation period. During the dormant season, we assumed the LAI 0.5 m2 m-2, based on the biophysical parameter table of
The albedo was derived from the 16-day estimates of the MODIS images. The missing values of albedo in winter were assumed as snow cover and were replaced by an albedo of 45% at the oak and 75% at the fallow plot (
Meteorological variables, soil water content and groundwater level were monitored by automatic equipment.
A weather station (iMETOS, Pessl Instruments, Austria) at the fallow plot and an automatic rain-gauge (Rainlog Data Logger, Rainwise, USA) 500 m apart from the oak plot were employed to monitor weather conditions (
The volumetric water content was monitored with FDR (Frequency Domain Reflectometers) using Decagon EC-5 probes (Decagon Devices, Pullman, USA) with a time interval of 15 min. The probes were installed at depths of 0.1, 0.3, 0.5 and 0.7 m.
Groundwater level was measured by a Dataqua DA-S-LRB 118 vented pressure transducer (Dataqua Elektronikai Kft., Balatonalmádi, Hungary) with time interval of 15 min. Manual groundwater level measurements were used to verify the reliability of the monitoring.
The Hydrus 1-D (
The soil depth was set to three meters at both plots and was partitioned into seven soil layers according to the soil sampling (0-20 cm, 20-40 cm, 40-60 cm, 60-80 cm, 80-100 cm, 100-120 cm, 120-300 cm). The Hydrus model computed the variables of water flow for 200 soil horizons with gradually increments by depth. During the modeling daily time steps were used.
Daily potential transpiration
The net groundwater supply
Water retention parameters and saturated hydraulic conductivity
The actual values of transpiration
Effective precipitation was computed by estimation of the canopy and litter interception loss at the oak plot. The canopy interception loss was calculated applying the Gash model in the growing season (for details, see
Litter storage capacity was set during the model calibration.
Interception loss
where
Potential transpiration
where
Leaf conductance
where
The reduction factors
Potential soil surface evaporation
Diurnal fluctuation of groundwater levels occurs in shallow groundwater regions. The main inducing factor is the diurnal changes of transpiration during vegetation periods (
where
The values of the minimum and maximum
Readily available specific yield was introduced by
Actual transpiration
where
Values of the parameter
where
Actual soil surface evaporation
The observed soil water content and groundwater level data were employed to calibrate the Hydrus model at both plots. The model was calibrated specifically for this two year period and not used for future simulation.
During the model calibration, the storage capacity of vegetation and soil hydraulic parameters were modified to reduce the deviations between the modeled and observed soil water content of each layer and groundwater level.
At the oak plot, first the interception loss of the dormant season was calibrated. The storage capacity of stems and branches was set to 0.5 mm (
In the fallow plot, the best fit with the observed soil moisture was achieved by using a storage capacity of 1.0 mm during the vegetation period and 0.5 mm for the period of the dormant season.
During the calibration of the saturated hydraulic conductivity, the initially computed values were modified for the root-zone (<1.5 m at the oak and <0.8 m at the fallow plot) of the soil profile. The calibrated
At the fallow plot, the calibrated
Readily available specific yield was calibrated to 0.032 at the oak and to 0.029 at the fallow plot.
The calibrated soil water content and groundwater levels were compared with measured values at both plots to check the performance of the model. The performance criteria were the coefficient of determination
The calibrated soil water contents compared well with observations at both plots (
The discrepancies at the beginning of the growing season of 2007 in measured
The calibrated groundwater levels compared quite well with measurements at both plots. Due to a malfunction of the pressure transducer, the continuous measurement failed at the fallow plot between November 2007 and June 2008. During this period regular manual groundwater depth measurements were employed to follow groundwater levels at the fallow plot (
Comparison of the measured and calibrated soil water content and groundwater levels showed no systematic divergence at the plots (
Total rainfall interception was twice as much in the forest than at the fallow plot considering the whole study period. In the 2007 growing season, 38% of the rainfall was intercepted at the oak plot, while at the fallow plot it was only 15% of the gross rainfall. As a consequence of more rainfall in 2008, the ratio of interception loss to precipitation decreased slightly at both plots.
According to the Hydrus model, the oak forest transpired approximately 33% more than the fallow vegetation, while groundwater consumption was three times higher during the study period.
Actual soil surface evaporation was only 4% of the total evapotranspiration at the oak plot, and 26% at the fallow plot during the whole study period. The low evaporation amount of the oak plot was the consequence of the high surface resistance due to the litter layer and the shading effect of the canopy. Available net radiation at the soil surface, and soil surface evaporation rates were higher at the fallow plot; thus the soil surface dried out more rapidly than at the oak plot.
Since weather conditions were quite contrasting during the growing seasons of 2007 and 2008, we decided to compare the water balances for both plots from 1st of April until 30th of September.
Groundwater consumption
In this growing season, the water uptake showed the effect of drought in summer. Until the start of the summer, the transpiration from the unsaturated zone
In the growing season of 2008 rainy weather provided a considerable amount of moisture for the unsaturated zone; thus the groundwater consumption was reduced from the preceding year. Groundwater consumption at the oak plot was 50% of the total transpiration, while at the fallow plot it was 25% in the growing season of 2008 (
While in the growing season of 2007 both vegetation covers relied significantly on groundwater resources, in 2008 the evapotranspiration loss of groundwater was reduced considerably due to the rainy weather at both plots.
The water balance components of the growing seasons of 2007 and 2008 are shown in
A meteorological tower was not set up at the oak plot, which contributed to the deviations of modeling results from the measurements. Air temperature and relative humidity at the fallow plot were applied above the canopy of the oak forest, which had an influence on the computation of potential transpiration and evaporation.
The empirical approach for computing
Water balance components of an oak and fallow plot were estimated from April 1st 2007 to April 1st 2009 by calibrating the Hydrus 1-D model using soil moisture and groundwater level measurements. The study period included a dry (2007) and a wet growing season (2008).
For the entire study period, the Hydrus 1-D model results have shown that the total transpiration of the fallow plot was only two thirds of that obtained in the oak plot, while the soil surface evaporation in the oak plot was approximately one fifth of that in the fallow plot. The separation of transpiration into unsaturated transpiration and groundwater consumption has revealed that the groundwater consumption at the oak plot was almost three times higher than at the fallow plot. The groundwater consumption was close to 60% of the total transpiration at the oak forest and approximately 30% at the fallow plot.
By comparing the dry (2007) and wet (2008) growing seasons, we found that groundwater consumption was approximately 40% less in the wet than in the drier growing season, despite the fact that the groundwater level was deeper during the dry period. Thus, during the dry season both vegetation covers relied considerably on the available groundwater resources.
The results obtained of this investigation reinforce previous reports on higher groundwater consumption of forests as compared to other vegetation covers. Therefore, future afforestation in arid regions with shallow groundwater levels should pay attention to the large groundwater depleting effect of forests, especially in light of future climate change and human water extraction.
This research was funded by National R+D (NKFP 3B/2002/012 and NKFP 6-47/2005), EU Joint Development (TÁMOP-4.2.2-08/1-2008-0020 and TÁMOP 4.2.2.B-10/1-2010-0018 “Talentum”) projects and the HAS Bolyai scholarship. The authors would also like to acknowledge NASA for providing the MODIS data free of charge, and anonymous reviewers for providing input to improve the quality of the manuscript.
Location of the study area.
Seasonal change of LAI (Oak plot: dashed line - MODIS data, solid line - applied).
Structure of the model. (PEFF): effective precipitation; (TP): potential transpiration; (T): actual transpiration; (TGW): groundwater consumption; (TUZ): transpiration from the unsaturated zone; (EP): potential soil surface evaporation; (E): actual soil surface evaporation; (QNET) net groundwater supply; (S): soil water storage. All components are in mm day-1.
Comparison of measured and calibrated soil water contents at the oak (A) and fallow (B) plots.
Comparison of measured and calibrated groundwater levels at the oak (A) and fallow (B) plots.
Comparison of the daily observed and calibrated soil moisture contents (A) and daily groundwater levels (B) at the oak plot.
Proportion of the water uptake from the unsaturated and saturated zone and precipitation at the oak (A) and at the fallow (B) plots during the growing season 2007.
Proportion of the water uptake from the unsaturated and saturated zones and precipitation at the oak (A) and at the fallow (B) plots during the growing season of 2008.
Main field-measured, estimated and calibrated parameter values of the oak and fallow plots. (l): Parameters estimated from literature; (d) parameters estimated from database analysis; (Oak): oak plot; (Fallow): fallow plot.
ParameterClass | Parameters | Unit | Oak | Fallow |
---|---|---|---|---|
Field-measured | Height | m | 20-25 | 0.1-0.2 |
Maximum LAI | m2 m-2 | 3.9 | 1.1 | |
Free throughfall coefficient | % | 14 | - | |
Root depth | m | 1.5 | 0.8 | |
Estimated | Albedod ( |
% | 10-16(75 at snow cover) | 11-17(45 at snow cover) |
Maximum leaf conductancel ( |
m s-1 | 0.0063 | 0.007 | |
Roughness lengthl ( |
m | 0.9 | 0.026 | |
Displacement heightl ( |
m | 15.12 | 0.11 | |
Light coefficient of extinctionl ( |
- | 0.5 | 0.5 | |
Stem flow ratio of precipitationl ( |
% | 3 | - | |
Densityd( |
trees ha-1 | 270 | - | |
Calibrated | Canopy storage capacity | mm | - | - |
Vegetation season | - | 1.17 | 1.0 | |
Dormant season | - | 0.5 | 0.5 | |
Litter storage capacity | mm | 0.5 | - | |
Saturated hydraulic conductivity | mm day-1 | - | - | |
Root-zone | - | 0.9 × 103 2.9 × 103 | 1.5 × 102 1.1 × 103 | |
Below the root zone | - | 1.4 × 102 | 0.8 × 102 | |
Readily available specific yield | - | 0.032 | 0.029 |
Mean of the three measured water retention data of the plots at six depths for five pressure heads. (Oak): Oak plot; (Fall): Fallow plot.
Pressure | Water content (%) | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
10 cm | 30 cm | 50 cm | 70 cm | 90 cm | 120 cm | |||||||
Oak | Fall | Oak | Fall | Oak | Fall | Oak | Fall | Oak | Fall | Oak | Fall | |
-1 cm | 52.7 | 34.4 | 42.5 | 38.4 | 48.5 | 39.4 | 46.9 | 39.7 | 37.9 | 38.3 | 37.0 | 37.2 |
-10 cm | 42.8 | 32.6 | 39.0 | 35.0 | 39.7 | 35.6 | 39.9 | 36.7 | 36.0 | 37.7 | 35.6 | 36.8 |
-333 cm | 20.2 | 19.4 | 20.1 | 19.3 | 17.8 | 23.7 | 22.5 | 30.3 | 32.3 | 30.5 | 23.4 | 33.9 |
-1000 cm | 16.7 | 15.1 | 16.0 | 14.5 | 15.5 | 18.3 | 18.0 | 22.0 | 25.9 | 22.7 | 16.8 | 26.4 |
-15000 cm | 7.8 | 10.4 | 7.9 | 9.3 | 7.5 | 11.9 | 7.3 | 14.0 | 14.1 | 14.0 | 9.6 | 17.4 |
Model performance criteria for soil water content and groundwater levels.
Plot | Criteria | R2 | RMSE% | ME |
---|---|---|---|---|
Oak Plot | Soil water content (10 cm) | 0.727 | 14.91 | 0.690 |
Soil water content (30 cm) | 0.830 | 11.95 | 0.796 | |
Soil water content (50 cm) | 0.787 | 13.52 | 0.769 | |
Groundwater level | 0.921 | 5.03 | 0.883 | |
Fallow Plot | Soil water content (10 cm) | 0.577 | 16.01 | 0.473 |
Soil water content (30 cm) | 0.667 | 10.83 | 0.472 | |
Soil water content (70 cm) | 0.828 | 9.83 | 0.782 | |
Groundwater level | 0.883 | 5.11 | 0.872 |
Water balance components (mm) of the growing seasons of 2007 and 2008 at the oak and fallow plots.
Water balance components | Growing season - 2007 | Growing season - 2008 | ||
---|---|---|---|---|
Oak plot | Fallow plot | Oak plot | Fallow plot | |
Precipitation ( |
261 | 261 | 383 | 401 |
Interception loss ( |
95 | 39 | 129 | 50 |
Soil surface evaporation ( |
22 | 139 | 41 | 154 |
Transpiration from unsaturated zone ( |
208 | 235 | 255 | 260 |
Groundwater consumption (TGW) | 405 | 144 | 255 | 87 |
Net groundwater supply ( |
289 | 125 | 184 | 55 |
Change of soil water storage ( |
-180 | -171 | -112 | -95 |