AGE 3116: Irrigation & Drainage Lecture Notes PDF

AGE 3116: Irrigation & Drainage Soil-water-plant relationships Lecture notes **THE HYDROLOGIC CYCLE** The hydrologic cycle describes the continuous movement of water on, above and below the surface of the earth. Water can change states among liquid, vapor and solid at various places in the water...

AGE 3116: Irrigation & Drainage Soil-water-plant relationships Lecture notes **THE HYDROLOGIC CYCLE** The hydrologic cycle describes the continuous movement of water on, above and below the surface of the earth. Water can change states among liquid, vapor and solid at various places in the water cycle. The water moves from one reservoir to another such as from river to ocean or from the ocean to the atmosphere by the physical processes of *evaporation condensation, precipitation, infiltration, runoff and subsurface flow.* *The hydrologic cycle* **[PHYSICAL PROCESSES/COMPONENTS OF THE HC]** 1. **PRECIPITATION** This is condensed water vapor that falls to the earth's surface. Most precipitation occurs as rain but also includes snow, hail, fog and sleet. Approximately 80% of annual global precipitation falls over the oceans. Rainfall is measured in millimeters (mm) by means of a rain guage. a. Characteristics of rainfall in sierra leone I. [Regional distribution (Annual]) North -- 200mm -- 2500mm Western Area -- 2500 -- 3500mm South/South -- 2500 -- 3000mm II. [Annual distribution] The distribution follows a MONOMODAL pattern ie. It has a simple peak Rf Max Drainage Rf min Jan July Dec b. Rainfall intensity I (mm/hr) This is the quantity of rain falling for a given time. Generally rainfall of high intensity has a short duration. 2. **INFILTRATION** This is the flow of water from the ground surface into the ground or the entry of water into the earth surf ace. Once infiltrated, the water becomes soil moisture or ground water. It constitutes the only source of water to maintain growth of plants. It is measured in millimeter per hour using an infiltrometer. The double-ring infiltrometer is preferred in most cases. Rainfall intensity varies even for a given rainfall event. [FACTORS THAT AFFECT INFILTRATION RATE] 1. Rainfall intensity, I (mm/hr) 2. Nature of soil surface 3. Texture of soil 4. Degree of vegetation cover on the soil 5. Soil moisture content at the time of infiltration 3. **PERCOLATION** Percolation is the movement of water through the soil and its layers by gravity and capillary forces. The prime moving force of groundwater is gravity. Water that is in zone of aeration where air exists in called VADOSE WATER. Water in the zone of saturation is called GROUND WATER. The boundary that segregates the vadose and saturation zones is called the WATER TABLE. 4. **RUN-OFF AND STREAM FLOW** Run-off is the movement of water, frequently from precipitation, across the earth's surface towards river channels, lakes, oceans or depressions.\` Run-off water can cause flooding and also erosion of top soil from agricultural land. Factors affecting run-off include; - Rainfall intensity, I - Size of catchment, A - Slope, soil type and ground cover (All considered as run-off coefficient, C. **Peak run-off rate, Qp (m^3^/s) = [C I A]** **360** Where Qp = peak run-off rate (m3/s) C = Run off coefficient I = Rainfall intensity (mm/hr) A = Catchment Area (ha) **MEASUREMENT OF STREAM FLOW** 1. [The continuity equation (Velocity/Area method)] The depends on measuring the average velocity of flow and the cross-sectional area of the channel and calculating the flow from: **Q (m^3^/s) = A (m^2^) x V (m/s)** The metric unit m^3^/s is referred to as Cumec. Because m^3^/s is a large unit, smaller flows are measured in litres per second (l/s). a. [FLOAT METHOD] This is a simple way to estimate the velocity. It involves measuring the time taken for a floating object to travel a measured distance. **WORKED EXAMPLE** Given that, length covered, l = 4m width of steam, w = 0.9m Depth of stream, d = 0.1m Average time, t av = 3+4/2 = 3.5min Calculate the stream flow. Velocity V = l/t av = 4/3.5 = 1.143m/min = 0.019m/s A = wd = 0.9 x 0.1 = 0.09m^2^ Q = AV = Float methods are only suitable for straight streams or canals where flow is fairly even and regular. 2. Use of weirs 3. The current meter **EVAPOTRANSPIRATION** The combination of two processes whereby water is lost on the one hand from the soil surface by evaporation and on the other hand from the crop by transpiration is referred to as ET. The Evapotranspiration rate is normally expressed in millimeters (mm) per unit time. The rate expresses the amount of water lost from a cropped surface in units of water depth. The time unit can be an hour, day, decade, month or even an entire growing period or year. The table below summarizes the units used to express the Evapotranspiration rate and the conversion factors. **Depth** **Volume per unit area** **Energy per unit area** --------------------------- ---------------- -------------------------- -------------------------- --------------------- **Mm day^-1^** **M^3^ ha^-1^day^-1^** **l s^-1^ ha ^-1^** **MJm^-2^ day^-1^** **1mm day ^-1^** 1 10 0.116 2.45 **1m^3^ ha ^-1^ day^-1^** 0.1 1 0.012 0.245 **1 s ^-1^ ha-1** 8.640 86.40 1 21.17 **1 MJ m^-2^ day ^-1^** 0.408 4.082 0.047 1 ***[ASSIGNMENT( should be submitted two weeks after day of lecture)]*** 1. ***[Describe the various ways by which humans have impacted the hydrologic cycle.]*** 2. ***[On a map,locate and briefly describe the agro-climatic regions of Sierra Leone]*** Soil--water-plant relationships **SOIL COMPOSITION** The soil is composed of three major parts: air, water and solids. The solid component forms the framework of the soil and consists of mineral and organic matter. The mineral fraction is made up of sand, silt and clay particles. The proportion of the soil occupied by water and air is referred to as the pore volume. **Schematic diagram of the soil as a three-phase system** **M~a~** =mass of air **, V~a~** = volume of air , **V~f~** = volume of pores **V~t~** = total volume **γ~b~** = dry bulk density = M~s~/V~t~ **W**= mass wetness= M~w~/M~S~ **θ**= volume wetness=V~W~/V~t~ = W**γ~b~** The pore volume is generally constant for a given soil layer but may be altered by tillage and compaction. The ratio of air to water stored in the pores changes as water is added to or lost from the soil. Water is added by rainfall or irrigation. Water is lost through *surface run-off, evaporation* (direct loss from the soil to the atmosphere), *transpiration* (losses from plant tissue) and either *percolation* (seepage into lower layers) or *drainage.* ***Soil moisture states*** The pore volume is actually a reservoir for holding water. However, not all of the water in the reservoir is available for plant use. All of the pores are filled with water immediately after heavy rainfall i.e. the soil is **saturated.** Gravity will pull some of the water down through the soli below the crop's root zone. The water that is redistributed below the root zone due to the force of gravity is **gravitational water.** After redistribution is complete, the soil is at **field capacity(θ~fc~)**. At this stage ,the large soil pores are filled with both water and air while the smaller pores are still full of water. At field capacity, the water and air contents of the soil are considered to be ideal for crop growth. Plants get most of their water from **capillary water**. This is the water retained in soil pores after gravitational water has drained. Surface tension (suction ) holds carpillary water around the soil particles. As water is removed by plants or by evaporation from the soil surface , the films of water remaining around the soil particles become thinner and are held by the soil particles more tightly. When the surface tension becomes high, the plant is unable to take up any of the remaining water, the soil water content has reached **permanent wilting point(θ~wp~)** **Total available water,TAW**, is the volume of water stored in the soil reservoir that can be used by plants.it is the difference between the volume of water stored when the soil is at field capacity and the volume still remaining when it reaches permanent wilting point. TAW is commonly expressed as the depth of water per unit depth of soil. Typical units are millimeter of TAW per meter of soil depth or cm/m. As a plant extracts water from the soil, the TAW decreases. The amount of TAW removed since the last irrigation or rainfall is the depletion volume. The management allowed deficit, MAD, is the degree to which the volume of water in the soil is allowed to be depleted before the next irrigation is applied. The volumetric moisture content at field capacity and permanent wilting point are: Θ~fc~ = γb W~fc~ Θ~wp~ = γb W~wp~ Where γb = dry bulk density ,W~fc~ = moisture content on dry weight basis at field capacity θ = volumetric moisture content = V~w~/ The total available water (TAW) is: TAW = (Θ~fc~- Θ~wp~) RD The Soil Moisture Deficit (SMD) is the depletion of the soil below field capacity at the time that particular soil moisture content, Θ, is measured. SMD = (Θ~fc~ --Θ)RD **Effective root depth (RD)** Rooting depth is the depth of the soil reservoir that the plant can reach to get the total available water (TAW). Crop roots do not extract water uniformly from the entire root zone. Thus, the effective root depth is that portion of the root zone where the crop extracts the majority of its water. Effective root depth is determined by both crop and soil properties. The effective root depth is the depth that should be used to compute the TAW. **Soil water balance** Evapotranspiration(ET) can be determined by measuring the various component of the soil water balance. This method consists of assessing the incoming and outgoing water flux into the crop root zone over some period. Irrigation (I) and rainfall (P) add water to the root zone. Part of P and I might be lost by surface runoff (RO) and by deep percolation (DP) that will eventually recharge the water table. Water might also be transported upward by capillary rise(CR) from a challow water table towards the root zone or even transferred horizontally by subsurface flow in (SF~in~) or out of (SF~out~) the root zone. Therefore ET can be deduced from the change in soil water content (ΔSW) ET = I + P --RO --DP + CR[±]{.math.inline} ΔSF[±]{.math.inline}ΔSW Worked problems 1.Using the table below, calculate TAW and SMD. Soil depth(mm) W~fc~ W~wp~ γb W ---------------- ------- ------- ------ ------ 0-250 0.25 0.12 1.35 0.15 250-600 0.28 0.15 1.55 0.19 600-925 0.22 0.10 1.45 0.16 925-1200 0.16 0.07 1.50 0.12 2\. The SMD for a particular soil, sampled 3 days after an irrigation event is 85mm. the rooting depth of the crop growing in the soil is 0.50m and Θ~fc~ is 0.35. What would be the volumetric moisture content of the soil at the time of sampling? 3\. The ratio MAD/TAW for a certain crop and soil is 0.55; the rooting depth of the crop 0.60 m and θ~wp~ and θ~fc~ of the soil are 0.18 and 0.42, respectively, daily ET~CROP~ is 5 mm. what is the length of time required for the soil moisture to be depleted to the level indicated? **MEASUREMENT OF SOIL MOISTURE** The methods and instruments available to evaluate soil water status may be classified in three ways. First, a distinction is made between the determination of water content and the determination of water potential. Second, a so-called direct method requires the availability of sizeable representative terrain from which large numbers of soil samples can be taken for destructive evaluation in the laboratory. Indirect methods use an instrument placed in the soil to measure some soil property related to soil moisture. Third, methods can be ranged according to operational applicability, taking into account the regular labour involved, the degree of dependence on laboratory availability, the complexity of the operation and the reliability of the result. Moreover, the preliminary costs of acquiring instrumentation must be compared with the subsequent costs of local routine observation and data processing. And there are direct and indirect methods of measuring soil moisture : i. +---------+---------+---------+---------+---------+---------+---------+ | *Sample | *Sample | *Depth* | *Bulk | *Weight | *Weight | | | time* | number* | | density | of wet | of dry | | | | | *(cm)* | * | sample* | sample( | | | | | | | | g)* | | | | | | *(g/cm^ | *(g)* | | | | | | | 3^)* | | | | +=========+=========+=========+=========+=========+=========+=========+ | *Before | *1* | *0-40* | *1.2* | *110* | *100* | | | irrigat | | | | | | | | ion* | | | | | | | +---------+---------+---------+---------+---------+---------+---------+ | | *2* | *40-100 | *1.5* | *96* | *130* | | | | | * | | | | | +---------+---------+---------+---------+---------+---------+---------+ | *After | *3* | *0-40* | *1.2* | *180* | *150* | | | irrigat | | | | | | | | ion* | | | | | | | +---------+---------+---------+---------+---------+---------+---------+ | | *4* | *40-100 | *1.5* | *156* | *150* | | | | | * | | | | | +---------+---------+---------+---------+---------+---------+---------+ | | | | | | | | +---------+---------+---------+---------+---------+---------+---------+ ii. **Neutron Probe** and extensive calibration required for each site. Nuclear International, Troxler Electronics, and Geoquip. iii. **Tensiometers** **Soil water potential** There are two basic methods of characterizing or measuring the water in the soil. The first is to measure the amount of water in the soil. An alternative to measuring the amount of water in the soil is to measure the energy state of the water. This approach leads to the soil condition water potential (New Mexico State University, 1999). Water potential measures the ability of soil water to move. Water potential is important to any process where soil water moves, such as infiltration and redistribution within the soil or the removal of water from the soil by evaporation and plant uptake. Water potential is the amount of work required per unit quantity of water to transport water in the soil. The four components of soil water potential are presented in the table below.