Groundwater uptake of vegetation in discharge regions is known to play an important role,
Significant afforestation is planned in Hungary in the coming decades, supported by the European Union (
Discharge and recharge regions of groundwater can be identified by changes of water table levels with depth, surface water networks and vegetation patterns (
Evapotranspiration (ET) by local phreatophytic vegetation is one of the primary processes of natural groundwater discharge. In discharge regions the plants often meet their water demands partly directly from the groundwater
In recharge areas where the deep-rooted vegetation is able to tap the groundwater, the typical diurnal water table fluctuation shows a stepped pattern. ET causes the water table to deepen during the day and then the water table remains relatively leveled or slightly deepens during the night. In such upland locations forests face a major threat by drought stress (
The higher water consumption of forests compared to grasslands and short rotation crops leads to the reduction of water yield (
A wide range of methods for estimating the actual ET are available (
Although the importance of groundwater uptake in shallow water table environments of the riparian zones is indisputable, the groundwater uptake of plant communities is rarely studied under various hydrologic conditions (
In this paper we analyzed the water table fluctuation and the ETgw of different vegetation covers (forest
In the frame of a national project (OTKA NN 79835 project), 108 plots of forested and nearby non-forested sites were investigated. In this paper two pairs of neighboring plots were compared in the Hungarian Great Plain in May-June of 2014. One study site is situated close to Nyírbogát (47 47′ N, 22 01′ E), where a 26-year-old poplar (
The area of Nyírbogát is built up from sandy river deposits of the early Pleistocene (
In contrast the Jászjákóhalma plots (wells F6 and F7) are located in local recharge zone, 3-4 m above the depression of the nearby draining ditch. The geological basis of this area is fluvial sediments, mainly sand and silt (
Meteorological conditions and groundwater level were monitored by automatic equipments. The weather stations (Vantage Pro2®, Davis Instruments Co., Hayward, CA, USA) recorded standard meteorological parameters (air temperature, relative humidity, global radiation, wind speed and precipitation) every five minutes. Groundwater wells were installed in each plot (F2, F3, F6, F7) with depth of 6 meters. Groundwater levels were measured by vented pressure transducers (Dataqua Elektronikai Kft., Balatonalmádi, Hungary) with time interval of 15 min.
At each site the mineral soil was sampled with 20 cm intervals down to 100 cm depth and with 50 cm intervals down to the full depth of the wells. Soil texture was determined using grain size distribution measured by the pipette method.
Diurnal signal of groundwater levels can be detected in areas with shallow groundwater depths due to the daily changes of transpiration during vegetation periods (
In case of the groundwater discharge area (Nyírbogát) the empirical version of a technique was applied (
The fundamental assumption was that the groundwater supply per unit area (
The ETgw rates were obtained as follows (
where
In the groundwater recharge area (Jászjákóhalma) where a step-like pattern of water table was observed, the following formula may be well suited for estimating ETgw (
The readily available specific yield was estimated from soil texture data. According to the trilinear diagram of
Rainfall events affect the groundwater uptake estimation through evaporation of intercepted rainfall from the canopy and evaporation loss from topsoil. These days were excluded since neither the rainfall interception nor the soil water content changes of the topsoil were measured.
The reference evapotranspiration of
During the study period the precipitation was only 30-50% of the average precipitation sum of May and June in both study sites (
The water table depression in Nyírbogát (0.37 m) was similar to the water drop observed by
In both regions the water table sank considerably more under the forest plantations (poplar: 0.15 m month-1; black locust: 0.125 m month-1) than under the corn plots (GW well F3: 0.1 m month-1; GW well F7: 0.075 m month-1).
In the discharge area the water table under the poplar plantation (GW well F2) increased at night, which combined with the daytime ET draw-down signal, producing the diurnal pattern observed in the hydrograph (
In the recharge area the diurnal fluctuation under the black locust forest (GW well F6) was considerably different showing clear step changes (about 0.4 cm). The lack of night-time recovery of water table meant that there was no net water supply, but the roots of the trees could tap the groundwater table for evapotranspiration (
It should be noted that the shape of the observed water table fluctuation is suitable to decide whether a particular location acts as a recharge or discharge zone. However, the groundwater zones are not only spatially variable but are also changing in time. The recharge area could transform to discharge area and
The net groundwater supply displayed a significant daily fluctuation changing between 0.04 and 0.1 mm/30 min under the poplar plot (GW well F2). In case of the black locust (GW well F6) there was no observed groundwater supply during night hours (
The calculated daily groundwater evapotranspiration of the poplar and black locust plantations were compared to PM_ET (Penman-Monteith reference evapotranspiration) for a period in May-June in 2014 (
Significant daily water table fluctuation suggested a substantial groundwater consumption of the poplar plantation of Nyírbogát. The ETgw of the poplar (mean ET rate: 3.6 mm day-1) plantation was about 65% of the potential evapotranspiration, calculated for the grass reference surface (5.18 mm day-1). On some days the groundwater ET was almost as high as the reference ET (calculated for a grass reference surface) showing that the higher leaf area index and canopy conductance of the forest canopy induced higher actual ET. The average ET of the black locust plantation was only 0.73 mm day-1, which was about 20% of the calculated daily potential ET (3.67 mm day-1).
The groundwater consumption of the studied plantations was less than observed in the various studies of the region, probably due to lower leaf area index and rooting depth.
The correlation coefficient between the reference ET and groundwater ET was not high at either plot (
ETgw of forest covers (poplar and black locust plantations) and adjacent corn-field plots were compared in discharge and recharge zones in the Hungarian Great Plains during a two-month period in 2014.
In the discharge zone the water table under the poplar plantation showed a diurnal pattern with night-time recovery. The roots of the poplar trees were able to reach the groundwater, thereby creating a water table depression and inducing groundwater supply in depths of 3.1-3.3 m. We applied a water table fluctuation method to estimate ETgw that incorporated the daily changes of groundwater supply. The mean ETgw of the poplar plantation was about 3.6 mm day-1. Contrary, the shallow roots of nearby corn field could not tap the groundwater, therefore neither daily groundwater fluctuation nor ETgw were detected.
The water table under the black locust plantation in the recharge zone displayed a step-like pattern, indicating the lack of night-time recovery of water table in similar water table depths of the poplar forest. ETgw was simply calculated using the observed water table changes and specific yield without the inclusion of the groundwater supply. The mean ETgw of the black locust plantation was 0.7 mm day-1, indicating the ability of groundwater extraction despite the upland location and the shallow rooting depth. Similarly to the discharge zone, ETgw was zero in the adjacent corn field.
This study indicates that local estimations of ETgw from simple water table measurements could assist to better understanding of groundwater use of varying vegetation types in recharge and discharge zones.
The research was supported by a fund from OTKA (NN 79835), the Postdoctoral Research Program of HAS No. PD-029/2015 and “Agrárklíma.2”(VKSZ_12-1-2013-0034).
Map of land use, elevation and location of monitoring plots with GW wells. Poplar plot well is indicated by F2, black locust plot well by F6 and the corn plots by F3 and F7.
Groundwater depths from the surface. Poplar forest (GW well F2) and nearby corn site (GW well F3) in Nyírbogát (A) and the black locust forest (GW well F6) and nearby corn site GW well F7) in Jászjákóhalma (B) (09.05.2014 - 31.06.2014).
Diurnal fluctuations of water table levels of the poplar (GW well F2) and the black locust (GW well F6) plots.
Net groundwater supply rate (
Daily ET rates of the poplar (GW well F2) and black locust (GW well F6) plantations.
Correlation coefficients (r) between reference ET and estimated ETgw values. (*): Correlation is significant at the 0.1 level (two-tailed); (**): correlation is significant at the 0.01 level (two-tailed).
Plot | r |
---|---|
Poplar (GW well 2) | 0.49** |
Black locust (GW well 6) | 0.35* |