Al-Azhar University-Gaza Master Program of Water and Environmental 

Science

I.  Introduction

A. Diagram

A ground water basin is defined volume of subsurface through which ground water flows from areas where the water table is recharged to location where ground water discharge occurs.

“ the water table is a subdued replica of the surface topography”

Regional groundwater flow

- Simple flow nets provide basis for understanding regional groundwater flow

- flow patterns are dictated by the variation in the shape of the basin and water table, and spatial patterns of hydraulic conductivity

Discharge area - water is crossing the water table upwards into unsaturated zone, land surface, or base of some surface water body

Natural basin yield - average rate of discharge from a hill slope or basin in the 

absence of anthropogenic disturbances

- water table is usually at or near the surface at the discharge area and are usually the

 sites of lakes, marshes, etc.

- the line separating recharge from discharge areas is called the hinge line of (or midline

Effect of Basin aspect Ratio

-basin shape exerts influence over the groundwater flow pattern

- the basin aspect ratio (length to depth) is one measure of basin shape

-two basins of identical length but different depths will have different natural basin yields

- the deeper basin will have a higher yield because there is more cross-sectional area to pass the water through

- the depth of the basin will also affect the pattern of flow- in deep basins vertical hydraulic gradient’s exist over a large portion of the basin- and shallower basins flow is essentially horizontal over most of a basinEffect of topography-Complex land-surface topography should produce complex water-table topography

-similarly local flow systems are superimposed on top of regional flow systems

-independently these flow systems appear similar but on different scales

-local vs. regional is subjective, but regional is usually considered to recharge at a basin divide and discharge at a valley bottom

-local flow system water has less of a residence time underground and therefore less interaction with geology

-local flow system water temperatures close to mean annual surface, more active in hydrologic cycle

-spring flows from local systems is more variable than regional

-intermediate flow systems have at least one local flow system between their recharge and discharge areas

-for shallow basins the regional system may exist but may be attenuation to as a result of the dominant influence of local flow systems

Stagnation points

II. Groundwater Flow Patterns

II. Groundwater Flow PatternsGW Divide

II. Groundwater Flow Patterns

GW Divide

Hinge Line

Recharge Zone Discharge ZoneDischarge Zone

III. Piezometer Patterns

IV. Effects of TopographyToth systems of flow

localintermediateregional

IV. Effects of Topography

IV. Effects of Topography

V. Flow System Mapping (recharge and discharge zones)

V. Flow System Mapping (recharge and discharge zones)

A. Topography

V. Flow System Mapping (recharge and discharge zones)

A. TopographyB. Piezometer Trends

V. Flow System Mapping

(recharge and discharge zones)

A. Topography

B. Piezometer Trends

C. Hydrochemical Trends

V. Flow System Mapping

(recharge and discharge zones)

A. Topography

B. Piezometer Trends

C. Hydrochemical Trends

D. Environmental Isotopes

V. Flow System Mapping

(recharge and discharge zones)

A. Topography

B. Piezometer Trends

C. Hydrochemical Trends

D. Environmental Isotopes

E. Soil, Vegetation and Land Surface Features

Groundwater-Lake/Wetland interactions

-hydrologic regime of a Lake is strongly influenced by the regional groundwater flow system in which it sets

- large permanent lakes almost always discharge areas for regional groundwater systems

- small permanent lakes in upland portions of watersheds usually discharge areas for local or intermediate flow systems

-where water table elevations are higher than Lake levels on all sides, recharge-lake only possible if high permeability at depth

Groundwater that is “noncyclical”- some groundwater by and large does not take part in flow systems:connate water: Water which was deposited, by geological means, simultaneously with the surrounding rock formations and held without flow. - This water usually occurs deep in the earth, and is high in mineral content due to long contact with rock.

connate water and fossil water often get interchanged

- some definitions have connate water as any water that is out of contact with the surface for a long time

magmatic water: may contain water that was subducted previously and juvenile water

VI. Salt Water Encroachment

A. The problem

VI. Salt Water Encroachment

B. Possible Solutions

1. modification of pumping pattern

VI. Salt Water Encroachment

B. Possible Solutions

2. artificial recharge

3. pumping troughs

4. freshwater ridge (injection barrier)

5. subsurface barrier

VI. Salt Water EncroachmentB. Possible Solutions

1. modification of pumping pattern2. artificial recharge3. pumping troughs4. freshwater ridge (injection barrier)5. subsurface barrier

Quantitative Interpretation of Regional Flow Systems (how much recharge/discharge)

-using flow net construction we can calculate rates are

recharge and discharge throughout the basin profile

Recharge-discharge profiles

-can be created above flow nets

-also a good way of checking flow that construction since in

steady state recharge must equal discharge

- 3-D maps possible as well