Modeling Water Budgets Research Articles

Evapotranspiration partitioning and its implications for plant water use strategy: Evidence from a black locust plantation in the semi-arid Loess Plateau, China

Evapotranspiration (ET) partitioning is crucial for understanding the impacts of ecophysiological and physical processes on water balance and plant water use strategy. Black locust (Robinia pseudoacacia) is one of the most widely planted species in the semi-arid Loess Plateau, China. The effects of its water use on stand soil water storage (S) and its water use strategy raised much concern due to soil water depletion and its growth degradation. In this study, meteorological factors, soil water content (SWC), ET components and net primary productivity (NPP) were observed in a black locust plantation during a drier (2015) and a wetter (2016) growing season. ET was 160.0 mm in 2015 and 255.0 mm in 2016, which accounted for 104.1% and 68.6% of the precipitation (P), respectively. Soil evaporation (E) accounted for the majority of ET (45.4–69.4% at monthly scale), while transpiration (T) was the smallest component of ET (9.7–28.8%). These results showed that it is E that consumed more soil water rather than T in the black locust plantation. E should be carefully considered and improved in predicting or modeling water budgets (e.g. soil water, ET partitioning and runoff) in the reforested catchments in this area. Additionally, the monthly T/ET decreased with P increasing, suggesting that black locusts could transpire water more effectively to survive under drier conditions with less P. Our study can not only improve the understanding of water budget in the reforested catchment in the semi-arid Loess Plateau but also provide the significant evidences of investigating the plant water use strategy from the point of ET partitioning.

Groundwater quality and management in arid and semi-arid regions: Case study, Central Eastern Desert of Egypt

This study presents a model budget for groundwater in the Central Eastern Desert of Egypt. The stable isotopic composition and hydrochemistry of groundwater samples collected from different aquifers were determined to identify recharge sources and water quality. Stable isotopic values suggest that shallow alluvial and fracture zone aquifers are recharged from seasonal precipitation, while groundwater in deeper sedimentary sub-basins is paleowater that was recharged during periods of less arid regional climate. Hydrochemical analysis indicates elevated salinity in each aquifer type, which is attributed to leaching and dissolution of terrestrial salts and to mixing with marine water. Groundwater from sedimentary sub-basin aquifers can be treated and used for drinking and domestic purposes. Groundwater from shallow alluvial and fracture zone wells is suitable for animal husbandry and mineral ore dressing. A model water budget shows that approximately 4.8×109m3 of recoverable groundwater is stored in sedimentary sub-basin aquifers, or approximately 550years of water at present utilization rates.

Michigan basin regional ground water flow discharge to three Great Lakes.

Ground water discharge to the Great Lakes around the Lower Peninsula of Michigan is primarily from recharge in riparian basins and proximal upland areas that are especially important to the northern half of the Lake Michigan shoreline. A steady-state finite-difference model was developed to simulate ground water flow in four regional aquifers in Michigan's Lower Peninsula: the Glaciofluvial, Saginaw, Parma-Bayport, and Marshall aquifers interlayered with the Till/"red beds," Saginaw, and Michigan confining units, respectively. The model domain was laterally bound by a continuous specified-head boundary, formed from lakes Michigan, Huron, St. Clair, and Erie, with the St. Clair and Detroit River connecting channels. The model was developed to quantify regional ground water flow in the aquifer systems using independently determined recharge estimates. According to the flow model, local stream stages and discharges account for 95% of the overall model water budget; only 50% enters the lakes directly from the ground water system. Direct ground water discharge to the Great Lakes' shorelines was calculated at 36 m3/sec, accounting for 5% of the overall model water budget. Lowland areas contribute far less ground water discharge to the Great Lakes than upland areas. The model indicates that Saginaw Bay receives only approximately 1.13 m3/sec ground water; the southern half of the Lake Michigan shoreline receives only approximately 2.83 m3/sec. In contrast, the northern half of the Lake Michigan shoreline receives more than 17 m3/sec from upland areas.

Biases of the observed atmospheric water budgets over the central United States

Analysis of the mean annual cycle of the observed water budget based on radiosonde data for the 20 year period 1973–1992 reveals a negative bias in the atmospheric estimate of mean moisture flux convergence over the central United States when compared to mean river discharge for the same period. Several possible sources of this bias are examined through the analysis of radiosonde, wind profiler, and river discharge data as well as water budget estimates from NCEP's mesoscale Eta Model. The analysis suggests that 20 years of radiosonde observations are sufficient to get stable mean water vapor flux convergence estimates. The analysis also shows that the bias between averaged vapor flux convergence and river discharge likely does not depend on changes in the locations of upper‐air observations or on changes in the radiosondes observational technique. Components of the atmospheric water budget diurnal cycle are estimated from hourly wind profiler data and from 3‐hourly Eta Model output. The analyses show that monthly mean atmospheric water budget quantities calculated from twice a day sampling, on average, significantly underestimate magnitudes of these quantities compared to estimates based on hourly or 3‐hourly analyses. The magnitude of the underestimation is more than 60% of the difference between mean observed terrestrial and atmospheric components of the hydrological cycle over the central United States, in agreement with earlier studies suggesting that temporal sampling may account for part of this bias. The effect of spatial sampling was evaluated using differences between atmospheric water budget terms from the Eta Model at its full 48 km resolution compared to Eta Model water budget estimates sampled at the radiosonde spatial resolution. These analyses show that the magnitude of the effect of spatial resolution is about 40% of the magnitude of the mean bias between the average moisture flux convergence and river discharge. The combined effect of temporal and spatial undersampling by radiosondes over the central United States is estimated to be 0.44 mm d−1. This value is within 8% of the estimated mean difference between river discharge and moisture flux convergence for the central United States.