Well hydraulics – Understanding groundwater flow and pumping in irrigation systems
📖 Introduction
In regions like Jammu and Kashmir, where surface water sources are often limited or seasonal, groundwater extraction through wells is a crucial component of irrigation. Well hydraulics is the branch of engineering that deals with the behavior of groundwater flow towards wells, discharge estimation, and the design of efficient well systems.
This topic is highly important for JKSSB civil engineering exams, where questions are commonly asked about aquifers, types of wells, and discharge formulas.
🌍 Understanding the Aquifer System
Groundwater is stored in aquifers – porous rock or sediment layers that can store and transmit water. The nature of the aquifer affects how water behaves when a well is pumped.
🔹 1. Unconfined Aquifer
- Also called water table aquifer.
- Water is under atmospheric pressure.
- The upper surface is the water table, which falls when water is pumped.
- Common in shallow wells and dug wells.
🔹 2. Confined Aquifer
- Sandwiched between two impermeable layers (clay or rock).
- Water is under pressure and may rise above the aquifer level when tapped.
- Used in artesian wells and tubewells.
🔹 3. Semi-confined Aquifer
- One layer is semi-pervious.
- Partial pressure condition.
- Water movement is slower than confined aquifers.
🔍 Types of Wells in Irrigation
1. Open or Dug Wells
- Shallow, large-diameter wells dug manually.
- Used in unconfined aquifers.
- Low yield and prone to contamination.
2. Tubewells
- Narrow, deep wells drilled using mechanical equipment.
- Can tap both confined and unconfined aquifers.
- High discharge, commonly used in agriculture.
- Requires proper design: strainer, casing, and pump.
3. Borewells
- Similar to tubewells but usually deeper.
- Smaller diameter than tubewells.
- Used for domestic and agricultural purposes.
4. Artesian Wells
- Water rises above ground level without pumping.
- Occur in confined aquifers under pressure.
💡 Basic Concepts in Well Hydraulics
1. Drawdown (S)
- The difference between the static water level and the pumping water level in a well.
- Represents the lowering of the water level due to pumping.
2. Cone of Depression
- A conical-shaped depression in the water table around the well due to pumping.
- The slope and radius depend on soil permeability and pumping rate.
3. Radius of Influence (R)
- The distance from the center of the well where the water table is unaffected by pumping.
- Beyond this, no drawdown is observed.
4. Well Yield (Q)
- Volume of water extracted per unit time.
- Expressed in liters/second or m³/day.
5. Specific Yield
- Amount of water drained from a saturated aquifer due to gravity.
6. Permeability (K)
- A measure of how easily water flows through soil.
- Units: cm/sec or m/day.
🧮 Discharge Formulas in Well Hydraulics
✅ Thiem’s Equation (for Steady Flow)
🧪 For Unconfined Aquifers:
Q=πK(h12−h22)ln(R/r)Q = \frac{\pi K (h_1^2 – h_2^2)}{\ln(R/r)}Q=ln(R/r)πK(h12−h22)
🧪 For Confined Aquifers:
Q=2πKb(h1−h2)ln(R/r)Q = \frac{2 \pi K b (h_1 – h_2)}{\ln(R/r)}Q=ln(R/r)2πKb(h1−h2)
Where:
- QQQ = Discharge
- KKK = Hydraulic conductivity
- h1,h2h_1, h_2h1,h2 = Head at two points in the aquifer
- RRR = Radius of influence
- rrr = Radius of the well
- bbb = Thickness of the aquifer
✅ Dupuit’s Assumptions:
- Flow is horizontal and radial.
- Water table slope is small.
- Darcy’s law is valid.
These assumptions simplify calculations and are applicable in real field conditions to a fair extent.
📌 Well Losses
Total drawdown in a pumping well consists of:
- Formation loss – due to flow in the aquifer.
- Well loss – due to flow through well screen and turbulence near the well.
S=BQ+CQ2S = BQ + CQ^2S=BQ+CQ2
Where:
- BBB = Formation loss coefficient
- CCC = Well loss coefficient
- QQQ = Discharge
🚜 Applications in Irrigation Engineering
- Well design based on yield requirement.
- Planning irrigation schedules based on discharge rate.
- Estimating pump capacity and selection.
- Preventing over-extraction and salinity problems.
- Planning conjunctive use of surface and groundwater.
⚠️ Limitations of Well Irrigation
- Overpumping can cause:
- Aquifer depletion
- Land subsidence
- Saline water intrusion
- Initial cost of drilling and equipment can be high.
- Inefficient design can lead to well failures.
📚 Well Hydraulics – Important for JKSSB Civil Exams
🔎 Expected Topics for JKSSB:
- Definition of aquifers
- Difference between confined and unconfined aquifers
- Drawdown and radius of influence
- Derivations of Thiem’s and Dupuit’s formulas
- Calculation-based numerical questions
- Practical examples of well irrigation in J&K
🧠 Conclusion
Understanding Well Hydraulics equips engineers and students with the knowledge to design, manage, and optimize irrigation systems, especially in groundwater-dependent regions like Jammu & Kashmir. For JKSSB aspirants, this topic is not just theory—it’s a stepping stone to solving real-world water management challenges.
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