Exhaustive Study Notes on Soil Conservation, Irrigation Technology, and Topographic Surveying

Introduction to Soil Science and Conservation Principles

Soil is defined as the earth’s fragile skin that anchors all life on Earth. It consists of a dynamic and complex ecosystem comprised of countless plant and animal species and is considered among the most precious resources for human existence. The importance of soil is multifaceted:

• Food and Cash Crops: Soil serves as the medium for growing essential agriculture for human consumption and trade.

• Livestock Support: Grasses and fodder trees necessary for livestock nutrition grow on the soil.

• Water Storage: Soil acts as a reservoir where water is available or stored.

• Microbial Habitat: It serves as a residence for microorganisms critical to nutrient cycling.

• Life Sustenance: It is the foundation for all forms of life on Earth.

• Infrastructure: All physical constructions are anchored in the soil.

Soil Erosion Processes and Causes

Soil erosion refers to the removal of valuable topsoil from land by natural physical forces or human activities. The process involves three distinct actions:

1. Soil Detachment: The initial removal of particles from the soil mass.

2. Movement: The physical transport of these particles.

3. Deposition: The settling of particles in a new location.
   Erosion is a natural process accelerated dramatically by human land use. It specifically relocates topsoil, which is high in organic matter, fertility, and soil life. While it can be a slow process that occurs unnoticed, it can also manifest at an alarming rate, leading to serious topsoil loss. Conditions that accelerate erosion include soil compaction, low organic matter, loss of soil structure, poor internal drainage, salinization, and soil acidity.

Primary Causes of Erosion

• Natural Forces: Wind, water, and temperature changes.

• Biological Factors: Activity by living organisms.

• Bad Farming Practices: These contribute to perpetual injury to the soil and include:

  • Ploughing up and down the slope.

  • Row cropping down the slope instead of along the contour.

  • Monocultures and shallow ploughing.

  • Growing crops in the wrong area or late planting.

  • Use of sledges.

Dynamics of Erosion

Erosion is a function of both erosivity and erodibility:

• Erosivity: The ability of raindrops to cause erosion.

• Erodibility: The susceptibility of the soil to erosion.

• Topography: Variables include the steepness and length of the land.

• Management: How the land is handled determines the degree of loss.

Classification and Forms of Soil Erosion

Classification by Origin

1. Geological Erosion: Includes soil-forming and soil-eroding processes that maintain a suitable balance for plant and animal growth.

2. Accelerated Erosion: Erosion resulting from human activities; this is the primary focus of soil conservation.

Classification by Agent

1. Water Erosion: The detachment and transportation of soil by water, including runoff from melted snow and ice. The process begins with splash erosion, where raindrop impact reduces infiltration by sealing soil pores, thereby increasing runoff.

2. Wind Erosion: Occurs in dry, bare soils. It involves three movement forms:

   • Suspension: Fine particles ($0.05$ to $0.10 \, ext{mm}$ diameter) are carried to great heights and over long distances.

   • Saltation: Particles ($0.10$ to $0.50 \, ext{mm}$) move in a series of low bounces ($300-900 \, ext{mm}$ high). This is the most important form and loosens the soil further.

   • Surface Creep: The largest particles ($0.50$ to $2 \, ext{mm}$) are rolled along the surface.

3. Gravity Erosion (Mass Movement): The downslope movement of soil including landslides, soil creep, and slumping.

Field Forms of Water Erosion

• Splash Erosion: The initial stage caused by raindrop impact.

• Sheet Erosion: Thin, uniform soil removal over a large area, progressively depleting the land.

• Rill (Finger) Erosion: Small, shallow channels formed by storm water concentrating in depressions. These are shallow enough to be removed by tillage.

• Gully Erosion: Deep channels that cannot be removed by normal tillage. They result from increased volume and velocity of runoff. Causes include animal trails, vehicle tracks, and destruction of vegetation in drainage ways. Effects include loss of fertile soil, siltation of dams, and land desiccation.

• Ephemeral Erosion: Temporary channels formed during heavy rain.

• Stream-bank Erosion: Enlargement of riverbeds by the undercutting and collapse of banks.

• Wave-action Erosion: Scouring of riverbanks or dam embankments by large waves.

• Slip Erosion: Slumping of crescent-shaped soil segments on grassy slopes, often exacerbated by overgrazing.

• Underground Erosion: Soil washing into underground canals, leading to surface collapse in sodic soils.

Estimating Soil Losses and Modeling

Soil loss is regarded as excessive when the rate of loss exceeds the rate of soil formation. In favorable conditions, it takes $12-40$ years to form $1 \, ext{mm}$ of topsoil. In Sub-Saharan Africa, the predicted natural formation rate is less than $5 \, ext{t/ha/year}$.

Universal Soil Loss Equation (USLE)

This model considers multiple factors to predict average annual soil loss: $A = R imes K imes LS imes C imes P$

• A: Amount of soil erosion ($t/ha$).

• R: Rainfall erosivity factor (based on kinetic energy, $KE = rac{1}{2} mv^2$).

• K: Soil erodibility factor (susceptibility based on texture, structure, and organic matter).

• LS: Slope length and steepness factor.

• C: Crop management factor (cover and tillage).

• P: Conservation practice factor.

Revised Universal Soil Loss Equation (RUSLE)

An improved version of USLE developed by the USDA in the 1990s. It uses improved databases, refined slope calculations, and accounts for seasonal variation in vegetation. While more accurate and suitable for GIS, it only estimates sheet and rill erosion and does not predict gully formation or sediment deposition.

Soil Loss Estimation Model for Southern Africa (SLEMSA)

Developed by HA Elwell in Zimbabwe to suit regional conditions (rainfall, soils, and farming systems). It consists of three sub-models: $Z = K imes C imes X$

• Z: Predicted mean annual soil loss ($t/ha/year$).

• K: Soil loss from bare soil ($30 \, m imes 10 \, m$ plot at 4.5 ext{%} slope).

• C: Ratio of soil loss from a cropped plot to bare fallow (Canopy sub-model).

• X: Topographic sub-model assessing slope length and steepness.

Universal Wind Erosion Equation (WEQ)

Estimates annual soil loss as a function of soil erodibility, surface roughness, climatic conditions, unsheltered field length, and vegetative cover. It is the wind erosion counterpart to USLE.

Principles and Objectives of Soil and Water Conservation

Principles

• Minimize Soil Disturbance: Keep soil intact to maintain structure and infiltration capacity.

• Protect Soil Surface: Maintain cover (vegetation, mulch) to reduce wind and raindrop impact.

• Reduce Runoff Velocity: Use mechanical structures like terraces and bunds to slow water.

• Increase Infiltration: Improve soil structure to allow water entry.

• Safe Disposal of Excess Runoff: Direct water through stable channels to prevent gullies.

• Maintain Soil Fertility: Protect nutrients and organic matter from being lost.

Objectives

• Prevention of Soil Erosion: Reduce topsoil loss.

• Sustainable Production: Maintain long-term land productivity.

• Water Conservation: Improve retention of rainfall and irrigation water.

• Sedimentation Reduction: Prevent siltation of rivers and dams.

• Flood Mitigation: Reduce peak runoff and downstream flooding.

• Environmental Protection: Protect biodiversity and ecosystems (critical in semi-arid areas like Namibia).

Conservation Agriculture (CA)

CA is an ecosystem approach to sustainable management based on three interlinked principles:

1. Continuous Minimal Soil Disturbance: No-till seeding and weeding to preserve soil structure.

2. Permanent Soil Cover: Maintaining crop biomass, stubble, or cover crops.

3. Cropping System Diversification: Rotations or associations including legumes.

• Note: A fourth suggested principle is Constant Traffic to manage soil compaction.

Hydrology and Runoff Estimation

Estimating runoff is essential for designing channels, drains, and culverts.

The Rational Formula

Used for small catchments to estimate maximum runoff rates: $Q = C imes I imes A$
(Where units are adjusted as needed; in some contexts, a conversion factor is applied to make the equation dimensionally consistent).

• Q: Rate of runoff ($m^3/s$).

• C: Dimensionless runoff coefficient (reflecting topography, soil, and vegetation).

• I: Rainfall intensity ($mm/h$).

• A: Catchment area ($ha$).

Cook's Method

Estimates runoff based on Catchment Characteristics (CC), which is the sum of three watershed values:

1. Cover: Heavy grass ($10$), Scrub ($15$), Cultivated ($20$), Bare/Eroded ($25$).

2. Soil Type/Drainage: Deep well-drained ($10$), Moderately pervious ($20$), Shallow/Impeded ($30$), Rocky/Medium clays ($40$), Impervious/Waterlogged ($50$).

3. Slope: Very flat/gentle ($5$), Moderate ($10$), Rolling ($15$), Hilly/Steep ($20$), Mountainous ($25$).
   Runoff is modified by the shape of the catchment:

• Square/Round: $1.0$

• Long and Narrow: $0.8$

• Broad and Short: $1.25$

US Soil Conservation Service (SCS) Curve Method

Estimates runoff yield ($Q$ in $mm$) using storm rainfall ($P$ in $mm$) and potential drainage/retention ($S$ in $mm$):
$Q = rac{(P - 0.2S)^2}{P + 0.8S}$
Values of $S$ depend on soil permeability (High: $50-150$, Medium: $25-100$, Low: $25-50$) and the number of days since the last storm.

Surveying and Topographic Mapping

Topographic surveying involves measuring horizontal and vertical positions of features to produce maps showing relief and elevation.

Surveying Procedures

1. Desk Study: Review maps, aerial photos, and satellite data.

2. Reconnaissance: Visit the site to identify boundaries, obstacles, and select benchmark locations.

3. Establishing Control Points: Setting permanent benchmarks as references for all measurements.

4. Measurement: Horizontal distances (tape, total station, GPS) and elevations (levels).

5. Recording: Maintaining a field book with sketches and measurements.

6. Data Processing: Calculating areas, elevations, and coordinates.

7. Plotting: Drawing boundaries and contours ($0.25$ to $2.0 \, m$ intervals).

8. Setting Out: Transferring plans back to the ground with pegs.

Levelling Specifics

• Definition: Determining height relative to a datum (usually mean sea level).

• Benchmark (BM): Fixed reference with known Reduced Level (RL).

• Backsight (BS): First reading on a known point.

• Foresight (FS): Last reading before moving the instrument.

• Height of Collimation Method:

  • $HI = RL + BS$

  • $RL = HI - (IS ext{ or } FS)$

• Rise and Fall Method: Determines the difference between consecutive points ($ext{Rise} = ext{Previous} - ext{Next}$). It is more accurate but time-consuming.

• Arithmetic Check: $extstyle rac{ extstyle\ extstyle extstyle extstyle extstyle extstyle extstyle extstyle extstyle extstyle extstyle extstyle extstyle extstyle extstyle extstyle extstyle extstyle}{ extstyle{BS}} - extstyle rac{ extstyle extstyle extstyle extstyle extstyle extstyle extstyle extstyle extstyle extstyle extstyle extstyle}{ extstyle{FS}} = ext{Last RL} - ext{First RL}$

Soil Moisture and Water Measurement

Moisture Measurement Methods

1. Tensiometric Method: Measures soil water suction. Standard types take $24 \, ext{hours}$ for reliability; quick-draw types take $5-10 \, ext{minutes}$. Working range is up to $0.85 \, ext{bars}$. Optimum suction for medium soils is $20-60 \, ext{centibars}$.

2. Thermogravimetric Method (Weight Basis):
   $P_w = rac{WS1 - WS2}{WS2} imes 100$ where $WS1$ is fresh weight and $WS2$ is oven-dry weight ($105^ ext{o} ext{C}$ for $48 \, ext{hours}$).

3. Volumetric Method (Volume Basis):
   $P<em>v = P</em>w imes BD$ (Bulk Density) or $P_v = rac{WS1 - WS2}{DW imes WS1} imes 100$.

Irrigation Water Measurement

• Units of Volume: Litre ($1/1000 \, m^3$), Cubic-metre ($1000 \, L$), Hectare-centimetre ($100 \, m^3$), Hectare-metre ($10,000 \, m^3$).

• Units of Flow: Litre per second ($L/s$), Cubic-metre per second ($m^3/s$).

• Velocity-Area Method: $Q = a imes v$. Velocity is measured using a float ($V = rac{ ext{Distance}}{ ext{Time}}$) multiplied by a coefficient (approx. $0.8$ to $0.9$) or with a current meter taken at $0.2$ and $0.8$ depth.

Irrigation Agronomy and Engineering

Crop Water Requirements (CWR)

CWR is the depth of water needed for evapotranspiration ($ET$). $ET$ is the sum of evaporation ($E$) and transpiration ($T$).

• Reference ET ($ET<em>o$): Potential ET of $10-15 \, ext{cm}$ tall cool-season grass.
  $ET</em>o = E<em>p imes K</em>p$ ($K_p$ is the pan coefficient, often $0.7$).

• Crop ET ($ET<em>{crop}$):
  $ET</em>{crop} = K<em>c imes ET</em>o$ where $K_c$ is the crop factor (varies from $0.3$ during the initial stage to $>1.0$ during the mid-season).

Irrigation Scheduling Terms

• Net Irrigation Requirement (NIR): Moisture to be supplied by irrigation to satisfy $ET$ plus leaching ($W<em>l$), minus effective precipitation ($R</em>e$) and soil storage.
  $NIR = ET + W<em>l - W</em>s - R_e$

• Gross Irrigation Requirement (GIR): NIR divided by system efficiency ($E<em>a$). $GIR = rac{NIR}{E</em>a}$

• Irrigation Interval (II): Days between irrigations. $II = rac{RAM}{Peak ET}$ (where $RAM$ is Readily Available Moisture).

• Design Capacity ($Q<em>c$): Required supply flow rate.
  $Q</em>c = rac{A imes d}{F imes H imes E_a}$ where $A = ext{Area}$, $d = ext{Net Depth}$, $F = ext{Irrigation Period}$, $H = ext{Operating Hours/Day}$.

Land Use Planning and Suitability Evaluation

Land Use Planning (LUP)

LUP identifies present situations and seeks options for utilization through stakeholder involvement. It occurs at National (1:1M scale), Regional (1:50k), and Local (1:5k) levels. Steps include establishing goals, analyzing problems, identifying opportunities, evaluating suitability, appraising alternatives, and implementing the plan.

FAO Land Suitability Classification

This system ranks land based on physical, chemical, and environmental limitations:

• Order S (Suitable): S1 (Highly Suitable - no limitations), S2 (Moderately Suitable - moderate limitations), S3 (Marginally Suitable - severe limitations).

• Order N (Not Suitable): N1 (Currently Not Suitable - limitations can be overcome), N2 (Permanently Not Suitable).

• Subclasses: Indicated by suffixes, e.g., $S2e$ (limitation of erosion hazard), $S2w$ (water limitation).

Soil Degradation and Management Issues

Major Degradation Processes

• Nutrient Depletion: Loss of nutrients from continuous cropping.

• Salinization: Salt accumulation due to poor irrigation.

• Compaction: Pore space reduction due to heavy machinery or livestock.

• Desertification: Degradation in arid regions leading to desert conditions.

• Waterlogging: Excess water reducing oxygen to roots.

Environmental Impact and Technology

• Negative Impact of Irrigation: Nutrient leaching, salt accumulation, and reduction of stream flows.

• Drone Technology: Modern applications for soil and field analysis, checking nutrient levels, moisture concentrations, and erosion detection.

• Water Harvesting: Alternatives to irrigation including storage in earth dams, sand sinks, and seepage dams.