Land and Water Engineering

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Land and Water Engineering

• Increasingly important for human survival
• Crucial for food security
• Demand for rural land use is rising, especially in developed areas

Challenges in Agriculture

• Finding balance between further development and the establishment of farming methods

Agriculture

• Cultivation of soil, harvesting of crops, and raising livestock

Agriculture in the Philippines

• Employs 22.36% of the Filipino workforce as of 2023 (World Bank)
• Issues include deforestation, water supply and quality, climate change, land conversion, poverty among farmers, and occupational hazards

Soil and Water Conservation Engineering

• Application of engineering and biological principles for soil and water management problems
• Implies utilization without waste

Soil and Water Conservation Problems

• Erosion Control
• Irrigation
• Drainage
• Flood Control
• Water Resources Development and Conservation

Erosion Control

• Different erosion types require specific mitigation strategies
• In the Philippines, 45.6% of land experiences moderate (28.3%) to severe (17.3%) erosion

Soil

• Vital resource for the production of renewable and nonrenewable resources

Soil Erosion

• Detachment and transport of soil by various factors like water, surface runoff, wind, gravity, earthquake, etc

Irrigation

• Artificial application of water to soil
• Can cause erosion
• Factors include soil type, slope, and flow rate

Irrigation Status in the Philippines (as of 2023, NIA)

• Estimated irrigable area of 3,128,631.00 ha
• Total service area of 2,155,026.23 ha
• NIS (National Irrigation Systems): 1,023,011.93 ha
• CIS (Communal Irrigation Systems): 740,596.60 ha
• PIS (Private Irrigation Systems): 203,381.21 ha
• Government agency-assisted: 188,036.49 ha

Agricultural Drainage

• Removal of excess water on the soil

Land Drainage

• Control of waterlogging and soil salinization

Drainage Problems in Flatlands

• Problems include water standing on the ground and waterlogging
• Possible solutions are leveling and smoothing the land, providing uniform slopes, and subsoiling to break hardpans

Subsurface Drainage Systems

• Required to maintain the groundwater table below the root zone in areas with insufficient natural drainage
• Discharge is through gravity flow, pumping, or tidal gates

Adverse Environmental Effect of Drainage

• Wetland conversion
• Water quality deterioration
• Landscape destruction

Flood Control

• Consists of overflow prevention on low land and the reduction of flow in streams

Flood

• Progressive abnormal increase in the elevation of the surface level of streamflow; most destructive calamities

Soil Water Conservation in Water-Short Regions

• Achieved through modified tillage and crop management techniques, level terracing, contouring, pitting, reservoirs, and other physical means of retaining precipitation on the land

Water Resource Development and Conservation

• Agriculture consumes 83% of available water
• 40% of irrigation water is lost through seepage, percolation, evaporation, and transpiration by phreatophytes
• Efficient water use is critical as demand rises

Soil and Water Conservation Technologies

Vetiver Grass Technology

• Hedgerows with deep root systems for erosion control
• Vetiver grass is densely tufted bunch grass

Small Farm Reservoir (SFR)

• A dam structure to collect rainfall and runoff designed for use in a single farm
• Typically has an area of about 300 - 1,500m²
• Serves about 0.5 to 1 hectare of farmlands
• Situated in gently undulating or flat terrain

Natural Vegetative Strips (NVS)

• Narrow live barriers vegetated with naturally occurring grasses and herbs

Trashlines

• Running heaps of plant biomass or residue materials erected across the slope along the contour
• Ideally 0.5m to 1.0m wide and 0.5m high

Residue Incorporation

• Increase the organic matter content of the soil to lessen the need for commercial fertilizer

Conservation Tillage

• Practice of planting seeds through the stubble of last season’s crop

Rockwell Terracing

• Done by piling the stones along the contour lines

Hydrologic Cycle

• Describes the storage and movement of water between the biosphere, atmosphere, lithosphere, and the hydrosphere

Evaporation

• Beginning of hydrologic cycle

Resulting Water Vapor

• Transported by moving air masses

Condensation/Sublimation

• Vapor undergoes to form clouds

Precipitation

• Fall to earth

Precipitation Dispersion

• Occurs through virga (evaporated while falling), interception then evaporation back into the atmosphere via throughfall
• Stored as ponds, puddles and surface water which are evaporated into the atmosphere
• Stored as snow and ice before melting or sublimation occurs after many years
• Flow over the surface (overland flow), discharge into streams and lakes (surface runoff), then evaporate or seep into groundwater
• Infiltration through the ground surface to join existing soil water and be removed by evaporation, throughflow, or downward percolation
• Groundwater component removed by capillary movement to soil surface or root zone to be returned to the atmosphere by evapotranspiration, or by groundwater seepage into surface streams and the oceans

Precipitation Defined

• Aqueous deposit, in liquid or solid form, falls from clouds.

Forms of Precipitation

• Mist: Particle size .005-.05mm. Large enough to be felt on the face when air is moving at 1 m/s.
• Drizzle: Particle size < 0.5mm. Small uniform drops fall from stratus clouds generally for several hours.
• Rain: Particle size 0.5-5mm. From nimbostratus or cumulonimbus clouds.
• Sleet: Particle size 0.5-5mm. Small, spherical to lumpy ice particles that form when raindrops freeze while falling through a layer of subfreezing air.
• Glaze: Particle size 1mm-2cm. Produced when super cooled raindrops freeze on contact with solid objects.
• Rime: Variable accumulations. Deposits usually consisting of ice feathers.
• Snow: Particle size 1mm-2cm. Crystalline natures of snow allow it to assume many shapes, including six-sided crystals, plates, and needles.
• Hail: Particle size 5mm-10cm or larger. Hard, rounded pellets or irregular lumps of ice.
• Graupel: Particle size 2mm-5mm. Forms as rime collects on snow crystals to produce irregular masses of “soft” ice.

Mechanisms of Precipitation

Coalescence

• Water droplet forms around a nuclei then falls at an increased velocity
• Droplet will break apart when its diameter reaches approximately 7 mm

Cooling

• Occurs when the amount of moisture in the atmosphere exceeds the saturation capacity of air
• Warm air can hold more water than cold air

Cooling Process

Adiabatic Cooling

• When air mass at a low elevation is lifted to a higher elevation

Frontal Cooling

• Happens along the border between a warm weather front and cold front

Contact Cooling

• Result of warm air blowing across a cold lake

Radiation Cooling

• When air is heated during day but cools during night

Types of Precipitation

Cyclonic Precipitation

• Lifting of air due to pressure difference

Frontal

• Two air masses clash with each other
  • Cold Front – cold air mass drives out warm air mass
  • Warm Front – warm air replaces cold air mass
  • Stationary Front – both air masses are drawn

Non-frontal

• Moist warm air is stationary, then moving cold air meets it

Convective Precipitation

• Warmer that its surrounding caused by upward movement; difference in temperature can result from unequal heating

Orographic Precipitation

• Caused by moist air masses which strikes some natural topographic barriers

Measurement of Precipitation

• Rain Gauge
  • Non-Recording – cylindrical gauges, ordinary rain gauges
  • Recording – siphon gauges, tipping bucket rain gauges

Rainfall Intensity

• Very intense storms are not necessarily more frequent in areas having a high total annual rainfall
• Storms of high intensity generally last for short periods and cover small areas

Storm Coverage

• Storms covering large areas are seldom of high intensity but may last for several days
• Infrequent combination of relatively high intensity and long duration storms gives large total amount of rainfall

Statistical Parameters Determination

• Including extreme value law and Log-probability law

Runoff

• Precipitation that reaches the surface but does not infiltrate the soil

Factors Affecting Runoff

• Rainfall
  • Intensity: Higher intensity = More runoff
  • Duration: Longer duration = More runoff
  • Areal Distribution: Larger coverage = Higher total runoff
  • Amount: More rain = More runoff (if soil is saturated)
  • Frequency: Frequent storms = Higher runoff

Mean Rainfall in the Philippines

• Varies from 965 to 4,064 mm annually
• Baguio City, eastern Samar, and eastern Surigao receive the greatest amount of rainfall
• Southern portion of Cotabato receives the least amount of rain

Watershed

• Also called catchment area, catchment basin, basin, drainage basin
• Area of land from which water drains into a river, stream, or other waterbody
  • Size: larger watersheds collect more rainfall, leading to higher total runoff volume but lower runoff per unit area
  • Smaller watersheds tend to generate flashier runoff responses
  • Orientation: parallel to prevailing storms receive rainfall more evenly, leading to a longer and more sustained runoff response
  • Perpendicular to storm movement experience concentrated runoff
  • Geology:
    • Soil texture, structure and depth
    • Soils with low infiltration rates have high runoff rates and volumes
    • Permeable rock formations allow greater infiltration
    • Impermeable rocks limit infiltration
    • Presence of groundwater storage reduces runoff
  • Shape: Long, narrow watersheds have lower runoff Topography
    • Flat or depressed areas without surface outlets have lower runoff than areas with steep, well-defined drainage patterns
  • Surface Culture
    • Forested and vegetated watersheds absorb rainfall and retards overland flow, increasing infiltration and reducing runoff
    • Urbanized areas prevent infiltration, causing high runoff and flash flooding
    • Agricultural areas may increase or decrease runoff depending on farming practices

Philippine River Basins

• 421 principal river basins
• 18 major river basins (e.g., Cagayan, Bicol, Mindanao, Agusan, Pampanga River Basin)

Estimating Peak Runoff Rates

• Design Runoff Rate: used to determine the capacity of the structure that will carry runoff
  • Temporary Structures: designed for runoff that will occur once in 10 years
  • Permanent Structures: once in 50 or 100 years

Time of Concentration

• Time required for water to flow from the most remote (in time of flow) point of the area to the outlet once the soil has become saturated and minor depressions are filled

Soil Conservation Service Method

• Developed for uniform rainfall using the assumptions for a triangular hydrograph

Runoff Volume

• Primary interest in the design of flood control reservoirs

Water Yield

• Also called annual runoff

Water Year

• Starts with the wet month in a given location

Hydrograph

• Graphical or tabular representation of runoff rate against time

Unit Hydrograph

• Hydrograph representing 1-inch of runoff

Triangular Hydrograph

• Used to approximate runoff hydrographs
  • Applied to subareas of a watershed
  • Time increments of rainstorms

Dimensionless Hydrograph

• Also called synthetic hydrograph
• Shape approximates the flow from an intense storm
• Has an arbitrary 100 units of flow for the peak and 100 units of time for the duration of flow

Design Hydrograph

• Developed from the dimensionless hydrograph by using appropriate conversion factors