Hydrology and Water Supply - Cheat Sheet
Your last-minute revision companion for the Rational Method, runoff calculations, water-supply demand, and hydrologic fundamentals. Every formula, definition, and pitfall needed in the final 30 minutes.
The Hydrologic Cycle & Fundamentals
Important facts
- Not all rainfall becomes runoff—infiltration, interception, and evaporation account for the remainder.
- Runoff coefficient C increases with imperviousness: grass → concrete → asphalt (0.1 → 0.95).
- The Philippine archipelago's tropical climate produces high rainfall but seasonal variation.
- Urban catchments have higher C and lower tₓ than rural catchments (faster peak runoff).
- Mixed catchments: use weighted-average C based on percentage of each land-use type.
Key definitions
Hydrologic Cycle
Example
Rainfall over a catchment; part infiltrates, part runs off into streams.
Continuous movement of water: precipitation → interception/infiltration → surface runoff → streamflow → evaporation/transpiration → precipitation.
Catchment (Watershed)
Example
A 100 km² mountainous basin feeding a river gauge station.
Area of land from which all water drains to a single outlet or monitoring point.
Runoff Coefficient (C)
Example
Pavement C ≈ 0.90; lawn C ≈ 0.15; mixed C = weighted average.
Fraction of rainfall converted to surface runoff; depends on land use and imperviousness (0.1–0.95).
Time of Concentration (tₓ)
Example
For a 10 km² urban catchment, tₓ ≈ 20–40 minutes.
Time for water to travel from the farthest point in the catchment to the outlet; determines design rainfall intensity.
Infiltration
Example
Sandy soil infiltrates faster than clay; affects runoff depth.
Process by which precipitation enters the soil; reduces runoff volume.
Diagrams to know
- Hydrologic cycle (evaporation, condensation, precipitation, infiltration, runoff pathways).
- Catchment map showing outlet, ridgeline, and contributing area.
The Rational Method — Peak Runoff Discharge
Formulas
Q = (C × i × A) / 360
Meaning
Q = peak discharge (m³/s); C = runoff coefficient (dimensionless); i = rainfall intensity (mm/hr); A = catchment area (hectares); 360 = unit conversion factor (SI practical units).
Watch out
The 360 factor is ONLY correct when i is in mm/hr and A is in hectares. If area is in m² or km², convert first or use Q = C × i × A with consistent base units (m/s, m/s, m²).
When to use
Always use for design peak discharge from small catchments (< 250 ha) when IDF data are available.
i = design intensity from IDF curve at duration = tₓ and return period T
Meaning
i = intensity (mm/hr) read from Intensity-Duration-Frequency curve for the locality; tₓ = time of concentration; T = design return period (e.g., 5-year, 10-year, 25-year storm).
Watch out
Do NOT use average intensity over the whole storm—use the intensity corresponding to duration = tₓ. IDF curves are locality-specific; use the correct city/region data.
When to use
Must be done before applying the Rational Method; intensity decreases as duration increases.
tₓ ≈ 0.06 × (L / √S)^0.77 [Kirpich formula, L in km, S in m/m]
Meaning
Empirical estimate of time of concentration; L = longest flow path; S = average slope.
Watch out
Kirpich is an approximation; actual tₓ depends on roughness and flow type. Use site-specific measured or design values when available.
When to use
Quick estimate when detailed tₓ data unavailable; typical results 10–60 min for small catchments.
Common values
Value
0.85–0.95
Symbol
C
Quantity
Runoff Coefficient, Pavement
Value
0.70–0.85
Symbol
C
Quantity
Runoff Coefficient, Asphalt Road
Value
0.40–0.70
Symbol
C
Quantity
Runoff Coefficient, Urban (Mixed)
Value
0.15–0.25
Symbol
C
Quantity
Runoff Coefficient, Lawn/Grass
Value
0.05–0.15
Symbol
C
Quantity
Runoff Coefficient, Forest/Dense Vegetation
Value
5–10 years
Symbol
T
Quantity
Design Return Period, Urban Drainage
Value
25–50 years
Symbol
T
Quantity
Design Return Period, Major Roads/Highways
Value
100 years (or greater)
Symbol
T
Quantity
Design Return Period, Critical Infrastructure
Important facts
- Rational Method assumes: (1) catchment area is uniform, (2) design storm covers entire catchment, (3) intensity is constant over duration = tₓ.
- Valid for small catchments (< 250 ha); overestimates for large areas with non-uniform rainfall.
- The method is deterministic (not probabilistic); return period is applied to rainfall intensity, not directly to Q.
- Peak discharge occurs when the entire catchment is contributing, i.e., when storm duration ≥ tₓ.
- IDF curves must be obtained from meteorological data or local water authority (PAGASA in Philippines).
- Rational Method is the industry standard for drainage design in the Philippines and most developed regions.
Key definitions
Peak Runoff Discharge (Q)
Example
A 20 ha catchment with C = 0.6 and i = 50 mm/hr yields Q = 1.67 m³/s.
Maximum rate of water flowing from a catchment during a design storm; used to size conveyance and storage.
Design Intensity (i)
Example
For a 10-year storm and tₓ = 30 min, read i = 60 mm/hr from the IDF curve.
Rainfall intensity (mm/hr) for a storm duration equal to the time of concentration and specified return period.
Return Period (T)
Example
A 25-year storm has a 4% chance of being exceeded in any given year.
Average interval (in years) between storms of equal or greater magnitude; inverse of annual exceedance probability.
Intensity-Duration-Frequency (IDF) Curve
Example
PAGASA or local water authority provides IDF curves for major Philippine cities.
Graph of rainfall intensity vs. storm duration for different return periods; locality-specific data.
Diagrams to know
- IDF curve family (multiple curves for different return periods; intensity decreases with duration).
- Runoff hydrograph: rising limb (0 to tₓ), peak at tₓ, recession.
Runoff Volume & Rainfall Depth
Formulas
V_runoff = C × P × A
Meaning
V_runoff = volume of runoff (m³ or ML); C = runoff coefficient; P = rainfall depth (m); A = catchment area (m²).
Watch out
Keep units CONSISTENT: if P is in mm, convert to m (÷1000); if A is in km², convert to m² (×10⁶) or ha (×100). Easy to drop a zero.
When to use
Calculate total runoff volume from a storm event; used for reservoir design and storage sizing.
Effective rainfall depth = C × P
Meaning
Portion of rainfall that becomes runoff; the rest infiltrates or evaporates.
Watch out
Do NOT confuse total rainfall P with effective rainfall C×P. Only the effective depth contributes to runoff.
When to use
When converting rainfall event to runoff volume for reservoir mass-balance or flood routing.
Common values
Value
150–300 mm in 24 hours
Symbol
P
Quantity
Typical monsoon rainfall depth, Philippines
Value
10 m³
Symbol
V
Quantity
1 mm rainfall on 1 hectare
Important facts
- Runoff volume depends on rainfall depth, catchment area, AND runoff coefficient—all three must be known.
- For rainfall events, volume is the primary input to flood-routing and reservoir-storage calculations.
- Conversion: 1 mm rainfall on 1 ha = 10 m³ runoff (useful mental math check).
- Effective rainfall (C×P) accounts for losses: infiltration, interception, depression storage, evaporation.
- In the Philippines, monsoon-driven peak rainfall depths (200–300 mm) are common; design accordingly.
Key definitions
Runoff Volume
Example
80 mm storm on 5 km² with C = 0.4 yields 160,000 m³ runoff (or 160 ML).
Total quantity of water discharged from a catchment during a rainfall event, equal to effective rainfall depth times area.
Rainfall Depth (P)
Example
A 100 mm storm deposits 100 L per m² (or 100 mm × 1 m² = 0.1 m³ per m²).
Vertical distance of water accumulated on a surface during a storm; measured in mm or m.
Megalitre (ML)
Example
A reservoir of 500 ML ≈ 500,000 m³.
Volume unit: 1 ML = 1 × 10⁶ L = 1,000 m³; commonly used in Australian and Philippine water reporting.
Diagrams to know
- Runoff coefficient layers: total rainfall P, loss (1–C)×P, effective runoff C×P.
- Storm mass-balance: rainfall volume in = runoff volume out + infiltration + evaporation + storage change.
Water Supply — Demand Estimation & Sources
Formulas
Q_avg = Population × per-capita daily consumption
Meaning
Q_avg = average daily demand (L/day or m³/day); population in persons; per-capita in L/person/day.
Watch out
This is the AVERAGE daily demand. Peak demands (max-day, peak-hour) are much larger and govern pipe/pump sizing. Do NOT use average to size emergency reserves or pressure systems.
When to use
Fundamental estimate for sizing water-supply infrastructure (source capacity, treatment, storage tanks).
Q_max_day ≈ 1.5 × Q_avg
Meaning
Maximum daily demand; typical factor for Philippine design is 1.5 (ranges 1.3–2.0 depending on season, code, and locality).
Watch out
Factor varies by code and locality. Always check local water-authority guidelines (MWSS, local DPWH) for the design factor—some areas use 1.2 or 2.0.
When to use
Size the main water transmission line and primary reservoir/storage tank.
Q_peak_hour ≈ 2.0–3.0 × Q_avg
Meaning
Peak hourly demand; typical factor is 2.5 (used to size distribution pipes, pumps, and local reservoirs).
Watch out
Peak-hour factor is more variable than max-day (2.0 to 3.5 depending on population density and water use patterns). Urban high-rise = higher factor; rural = lower factor.
When to use
Design the distribution system, local storage reservoirs, and pump capacities.
Storage volume = (Q_max_day - Q_avg) × time interval + emergency reserve
Meaning
Volume needed in elevated tank or reservoir to buffer peak demands above average inflow.
Watch out
Emergency reserve (e.g., 1 day of average demand) must be added for system reliability and firefighting.
When to use
Size elevated storage tanks to meet peak-hour demands when source flow is constant.
Common values
Value
150–200 L/person/day
Symbol
c
Quantity
Typical per-capita daily consumption, urban Philippine
Value
100–120 L/person/day
Symbol
c
Quantity
Typical per-capita daily consumption, rural Philippine
Value
180–200 L/person/day
Symbol
c
Quantity
Design per-capita for master planning, Philippines
Value
1.5 (range 1.3–2.0)
Symbol
f_max_day
Quantity
Maximum day factor
Value
2.5 (range 2.0–3.0)
Symbol
f_peak_hr
Quantity
Peak hour factor
Value
20–50%
Symbol
UFW
Quantity
Unaccounted-for water (UFW) in developing systems
Important facts
- Water demand = population × per-capita consumption; demand increases with urbanization and living standard.
- Peak demands are 1.5–3.0× average; pipes, pumps, and storage must be sized for peak.
- Philippine Code/MWSS typically uses max-day factor 1.5 and peak-hour factor 2.5–3.0.
- Seasonal variation: higher demand in dry season; design must account for year-round reliability.
- Unaccounted-for water (UFW) in older Philippine systems can exceed 35%; water-loss audits are critical.
- Emergency reserve (typically 1 day of average demand) must be maintained in elevated tanks for firefighting and service outages.
- Population growth rate affects future demand: design should consider 5–20 year projection (per PRC and RA 544 requirements).
Key definitions
Per-Capita Daily Consumption
Example
Manila urban: ~180 L/c/day; provincial: ~100–120 L/c/day.
Water use per person per day (L/person/day); includes domestic, commercial, industrial, and losses; typical range 100–250 L/c/day.
Average Daily Demand (Q_avg)
Example
10,000 people at 150 L/c/day = 1.5 ML/day average.
Mean water consumption from a population or service area, calculated as population × per-capita.
Maximum Day Demand (Q_max_day)
Example
Average 1.5 ML/day → max-day 2.25 ML/day.
Highest daily water use; typically 1.5× average (occurs on hot, dry, or peak-season days).
Peak Hour Demand (Q_peak_hour)
Example
Average 1.5 ML/day (62.5 L/hr on average) → peak-hour demand up to 150–190 L/hr (at 2.5× factor).
Highest hourly consumption; typically 2.0–3.0× average; occurs during morning/evening use.
Unaccounted-for Water (UFW)
Example
If source supplies 1000 m³/day but only 750 m³ is billed, UFW = 25%.
Water lost to leakage, metering errors, and theft; can be 20–50% of total supply in developing systems.
Diagrams to know
- Demand hydrograph over 24 hours: peak in morning (6–8 am) and evening (6–8 pm), trough at night.
- Demand hierarchy: average → max-day (×1.5) → peak-hour (×2.5).
Water Supply Sources: Surface & Groundwater
Formulas
Well yield equation: Q = (2π k h Δh) / ln(R/r)
Meaning
Q = well discharge (m³/s or m³/day); k = hydraulic conductivity (m/s); h = saturated thickness of aquifer (m); Δh = drawdown (m); R = radius of influence (m); r = well radius (m).
Watch out
This is a simplified steady-state formula (Thiem equation). Real wells show time-dependent drawdown (Theis); transient solutions and step-drawdown tests provide more accurate yield. Δh and R must be measured or estimated carefully.
When to use
Estimate groundwater well capacity from aquifer properties and drawdown; foundational for well design.
Reservoir mass balance: Inflow − Outflow = ΔStorage
Meaning
Over a period, total inflow (runoff, tributary, rainfall) minus outflow (demand, spillage, evaporation) equals change in reservoir volume.
Watch out
Evaporation can be significant in tropical climates (3–5 mm/day); must be included in the mass balance. Seasonal variation is critical: rainy season fills reservoir, dry season depletes it.
When to use
Size a reservoir to ensure reliable supply during dry season; plot cumulative inflow vs. cumulative demand to find required storage.
Yield = firm yield + secondary yield
Meaning
Firm yield = guaranteed supply during driest year; secondary yield = surplus available in wet years (may be spillage or additional use).
Watch out
Design demand should NOT exceed firm yield; secondary yield is bonus but not guaranteed supply. Over-allocation is a common cause of supply failure in the Philippines.
When to use
Assess the safe, reliable capacity of a source (well or reservoir).
Common values
Value
10–50 m³/hr (few wells exceed 100 m³/hr)
Symbol
Q
Quantity
Typical groundwater well yield, Philippines
Value
1–100 m/day
Symbol
k
Quantity
Hydraulic conductivity, sandy aquifer
Value
0.01–1 m/day
Symbol
k
Quantity
Hydraulic conductivity, clay aquifer
Value
2–10 m
Symbol
Δh
Quantity
Typical drawdown in production well
Value
600–900 mm/yr
Symbol
R
Quantity
Groundwater recharge, wet Philippine zones
Value
100–300 mm/yr
Symbol
R
Quantity
Groundwater recharge, dry Philippine zones
Value
3–5 mm/day
Symbol
E
Quantity
Evaporation from reservoir, tropical
Important facts
- Surface water sources require intake structures (dams, weirs), treatment plants, and transmission lines; capital-intensive but can supply large populations.
- Groundwater is preferred in rural/provincial areas; wells are decentralized and low-cost but yield is limited per well (typically 10–50 m³/hr).
- Coastal aquifers are vulnerable to seawater intrusion if overpumped; monitor salinity in well water.
- Reservoir sizing requires analysis of historical streamflow and rainfall; seasonal storage is critical in monsoon climates.
- The Philippines' hydrologic year is Nov–Oct (dry season Dec–May); design reservoirs to bridge the gap.
- Groundwater recharge in the Philippines is highly variable: ~900 mm/yr in wet zones, <300 mm/yr in dry zones.
- Well interference: multiple wells in one aquifer reduce individual yields; aquifer testing (step-drawdown) quantifies this effect.
- Saltwater intrusion in coastal wells is a major challenge; requires protective drilling depths and monitoring.
Key definitions
Surface Water Source
Example
Laanan River reservoir supplying Metro Manila.
Water from rivers, streams, or reservoirs; advantages: large yield, simpler intake; disadvantages: high treatment cost, seasonal variation, flooding/contamination risk.
Groundwater Source
Example
Deep groundwater wells in provinces; typical yield 10–50 m³/hr per well.
Water from aquifers accessed via wells; advantages: generally clean, stable yield; disadvantages: lower yield per well, aquifer depletion risk, salinity (coastal areas).
Aquifer
Example
Piezometric surface rises and falls with recharge; overpumping causes permanent depletion.
Underground layer of rock or soil that stores and transmits groundwater; defined by hydraulic conductivity and storage coefficient.
Well Drawdown (Δh)
Example
Pumping 20 m³/hr from a well may cause 2–5 m drawdown depending on aquifer properties.
Vertical distance between static water level and pumping level; increases with discharge; indicates aquifer response.
Firm Yield
Example
A reservoir with 1 year's storage can supply its average inflow as firm yield during extended drought.
Guaranteed, sustainable water supply from a source (well or reservoir) during the driest year on record; used for safe design.
Safe Yield (Sustainable Yield)
Example
Aquifer recharge = 50 m³/day → safe yield ≤ 50 m³/day (avoid lowering water table).
Long-term average groundwater extraction rate that does not exceed recharge; prevents aquifer depletion.
Diagrams to know
- Cross-section of aquifer: confined (artesian) vs. unconfined (phreatic); recharge zones and discharge zones.
- Seasonal hydrograph of a river: peak flow in wet season (Aug–Oct), minimum in dry season (Apr–May).
- Reservoir water-balance diagram: inflow, demand, spillage, evaporation, seepage.
Design Practice & Code Requirements (PRC/NSCP/RA 544)
Common values
Value
5–10 years
Symbol
T
Quantity
Design return period, urban storm drainage
Value
10–25 years
Symbol
T
Quantity
Design return period, road/highway drainage
Value
50–100 years
Symbol
T
Quantity
Design return period, major infrastructure
Value
1.5–3.0% per year (varies by region)
Symbol
r
Quantity
Population growth rate used in design, Philippines
Value
20–30 years
Symbol
n
Quantity
Design period (planning horizon), water supply
Important facts
- RA 544 requires all water-supply and major drainage designs to be prepared and sealed by a licensed Professional Civil Engineer.
- NSCP 2015 aligns with international standards (ACI, AISC) and is the mandatory code for structural and civil works in the Philippines.
- Design storms: PAGASA (Philippine Atmospheric Geophysical and Astronomical Services Administration) provides official IDF curves for all major cities.
- Rational Method is accepted by NSCP and is the standard for preliminary/detailed design of drainage and stormwater systems.
- Water-supply design must account for 20–50 year population growth (per PRC and RA 544 guidelines).
- Minimum per-capita design is 150–200 L/person/day in urban areas; rural may be lower (~100 L/c/d).
- All water-supply infrastructure must include emergency reserve (typically 1 day of average demand) and system redundancy.
- Environmental impact assessment (EIA) is mandatory for all water-supply and major drainage projects (RA 8749, Clean Air Act; RA 6969, Toxic Substances).
Key definitions
RA 544 (Civil Engineering Law, 1974)
Example
All major water-supply and drainage designs must be stamped by a licensed Professional Civil Engineer (PEng).
Philippine law regulating the practice of civil engineering, including water-supply system design. Requires licensed engineers; sets competency standards.
NSCP 2015 (National Structural Code of the Philippines)
Example
NSCP Chapter 3 specifies return periods for drainage (5–10 yr) and critical infrastructure (25–100 yr).
Philippine design standard for building and infrastructure; includes water-supply and drainage sections referencing design intensity, return periods, and safety factors.
Design Return Period (Recurrence Interval)
Example
Urban drainage: 5–10 yr; highways: 25–50 yr; dams: 100–1000 yr.
Expected frequency of a design storm; chosen based on structure importance and risk tolerance (higher return period = more expensive design).
Factor of Safety / Design Factor
Example
Peak-hour demand multiplied by 1.2–1.5 factor for design; well yield reduced by 0.5–0.7 factor for safe yield.
Multiplier applied to calculated demand or runoff to account for uncertainty, growth, and variability; ensures adequate capacity.
Diagrams to know
- Return period vs. design intensity relationship: lower return period = lower intensity requirement = smaller (cheaper) pipes.
- Design flow hierarchy: average → max-day → peak-hour → extreme event (e.g., 100-year storm).
Must remember
- Rational Method formula: Q = (C × i × A) / 360 — the 360 factor is MANDATORY when i is in mm/hr and A is in hectares; always include it.
- Intensity i must be read from the IDF curve at duration = time of concentration (tₓ), NOT at some other duration or as an average.
- Runoff coefficient C increases with imperviousness: lawns ~0.2, asphalt ~0.75, concrete pavement ~0.9; for mixed land use, use weighted average.
- Water-supply design requires THREE demand levels: average (baseline), max-day (~1.5×), and peak-hour (~2.5×); pipes and storage must be sized for peak.
- Runoff volume V = C × P × A; keep units consistent (mm→m, km²→m²); 1 mm rain on 1 ha = 10 m³ runoff (mental-math check).
- Return period (design storm recurrence interval) is set by structure importance: urban drainage 5–10 yr, highways 25–50 yr, critical infrastructure 50–100 yr.
- Surface water requires large capital investment (dam, treatment, transmission) but supplies large populations; groundwater is cheaper but lower yield per well (~20–50 m³/hr typical).
- Reservoir sizing uses mass balance: cumulative inflow vs. cumulative demand over the driest year on record; storage capacity = difference between peak deficit and supply.
- RA 544 and NSCP 2015 are mandatory in the Philippines; all major water supply and drainage must be designed and sealed by a licensed Professional Civil Engineer.
- Unaccounted-for water (UFW), salinity intrusion (coastal), and aquifer depletion (groundwater) are real design constraints; account for 20–30% UFW and avoid overpumping safe yield.
Last minute tips
- In exam: If Rational Method answer seems too large or too small, CHECK THE 360 FACTOR. Most common error is calculating Q without dividing by 360 when i is in mm/hr and A is in ha.
- IDF curves are location-specific—look at the city/region given in the problem and find the matching curve. If not provided, estimate intensity based on typical values (50–100 mm/hr for 5–10 yr storms).
- When converting areas: 1 km² = 100 hectares = 10⁶ m²; write it out step-by-step to avoid off-by-a-factor-of-10 errors. Same for rainfall depth: mm → m requires ÷1000.
- Water-demand multipliers are exam favorites: remember 1.5× for max-day and 2.5× for peak-hour. If the question asks for peak-hour and you forget the factor, you'll be 2.5× too low.
- Always sketch a runoff or demand hydrograph if time permits—visualizing the peak helps catch errors. Rational Method peak occurs at t = tₓ; demand peaks occur early morning and evening.
Comparison tables
Surface Water vs. Groundwater Sources
| Characteristic | Surface Water | Groundwater |
|---|---|---|
| Typical yield per source | High (reservoirs: 10⁶+ m³/day; rivers: variable) | Moderate (wells: 10–100 m³/day typical) |
| Water quality (raw) | Poor to fair (turbid, contaminated); requires treatment | Good to excellent (filtered by soil); minimal treatment |
| Capital cost | High (dam, intake, treatment plant, transmission) | Low (well drilling, simple pump house) |
| Seasonal variation | High (flooding in wet season, low flow in dry season) | Low (more stable; recharged slowly) |
| Vulnerability to drought | High (reservoir can deplete in extended dry period) | Moderate (depends on aquifer thickness and recharge) |
| Contamination risk | High (exposed to surface pollution, runoff, sewage) | Low (protected by soil layers); but salinity risk in coastal areas |
| Suitable scale | Large cities, regional systems (centralized) | Small towns, rural, distributed supply |
| Depletion concern | Limited if sustainably managed (watershed recharge) | High if overpumped (aquifer depletion) |
Runoff Coefficient (C) by Land Use
| Land Use Type | C Range | Description |
|---|---|---|
| Dense forest / vegetation | 0.05–0.15 | Very permeable; high infiltration. |
| Grassland / lawn | 0.15–0.30 | Permeable; moderate infiltration. |
| Cultivated agricultural land | 0.20–0.40 | Loose soil, fair infiltration. |
| Residential (low density, yards) | 0.30–0.50 | Mixed grass and buildings; moderate runoff. |
| Residential (medium to high density) | 0.50–0.75 | Significant impervious area (roofs, streets). |
| Commercial / industrial zone | 0.60–0.80 | Large parking lots, buildings; high runoff. |
| Asphalt road / dark pavement | 0.70–0.85 | Nearly impervious; minimal infiltration. |
| Concrete pavement / parking | 0.85–0.95 | Fully impervious; maximum runoff. |
Demand Multipliers & Peak Factors
| Demand Type | Typical Factor (×Avg) | Design Application |
|---|---|---|
| Average daily demand | 1.0 | Baseline for all calculations. |
| Maximum day demand | 1.3–2.0 (typically 1.5) | Size main transmission line & primary storage. |
| Peak hour demand | 2.0–3.5 (typically 2.5) | Size distribution pipes, local tanks, pumps. |
| Emergency / fire reserve | ~1.0 day of average | Maintain in elevated tanks for system reliability. |
Rational Method — Pitfall Checker
| Common Mistake | Correct Approach | Impact if Wrong |
|---|---|---|
| Forgetting the 360 unit-conversion factor | Use Q = (C × i × A) / 360 when i in mm/hr, A in hectares. | Overestimate Q by ~360×; design becomes oversized or fails. |
| Using wrong intensity (not at duration = tₓ) | Always read intensity from IDF curve at the design storm duration = tₓ. | Wrong intensity → wrong Q; underestimate or overestimate runoff. |
| Applying average rainfall intensity over the whole storm | Use intensity for duration = tₓ (not storm average). | Peak Q is underestimated; drainage fails. |
| Converting area wrong (km² to ha) | 1 km² = 100 ha; multiply by 100, not 10. | Off by factor of 10 in Q. |
| Forgetting to estimate or verify tₓ | Use Kirpich formula or measure from site; affects intensity selection. | Intensity mismatch → incorrect Q. |
| Using design demand (peak-hour) as average | Keep demand types separate: Avg, Max-day, Peak-hr. | Oversize or undersize reservoir and pipes. |
| Not accounting for unaccounted-for water (UFW) | Add UFW (~30%) to design demand: source = demand / (1 − UFW). | Source undersized; insufficient supply to customers. |
| Ignoring seasonal variation (dry vs. wet season) | Check water balance over full hydrologic year; size reservoir to bridge dry season. | System fails during drought (dry season). |
Diagrams
Hydrology & Water Supply — Concept Map
Comprehensive mind map showing the hierarchical relationship between hydrologic concepts, the rational method components, water-supply demand estimation, and design standards. Use this to quickly orient yourself on major topics and their interconnections.
Rational Method Problem-Solving Flowchart
Step-by-step flowchart for solving rational method problems. Follow this in exams to avoid missing the 360 factor, wrong intensity, or unit errors. Decision points ensure you verify land use, intensity selection, and unit consistency.
Water Supply Demand & Storage Design Procedure
Procedural flowchart for water-supply design from demand estimation through source selection and storage sizing. Includes the critical step of accounting for UFW and emergency reserve; required for meeting RA 544 and NSCP 2015 standards.
Demand Variation Over 24 Hours (Typical Urban Pattern)
State diagram showing typical 24-hour demand cycle in urban areas. Morning and evening peaks (2.5× average) occur due to bathing, cooking, and cleaning; night demand drops to ~0.3×. Critical for understanding why pipes and pumps must be sized for peak-hour, not average demand.
Rainfall Mass Balance on a Mixed Catchment
Pie chart illustrating how 100 mm of rainfall is partitioned on a mixed urban catchment (C=0.5): 50% becomes runoff, 30% infiltrates into soil, 15% evaporates, 5% is retained in surface depressions. Emphasizes that only the C×P fraction contributes to runoff volume.