Lecture5 soil and infiltration dung

VFU-2016 Lecture #5 Soil and Infiltration Principles of Watershed management Dr Bui Xuan Dung- Department of Environment Management Importance of Soils  Key principle of watershed management is to maximize infiltration and minimize surface runoff;     Minimizes surface erosion; Lower peak flows and less flooding; More water for plant growth; Need to understand how water and soils interact (“soil physics”) for both land management and resource protection! Soil generation Weathering of bedrock material Alluvial sediment transport (vận chuyển chất lắng đọng bồi tích) SOILS CONTENTS Soils = parent material + organics + water + air soil horizons - distinct soil layers soil profile - sequence of layers Use of a double-ring is often assumed to minimize the edge effects “ the infiltration from the inner cylinder is also very much affected by divergence and edge effects Field studies also have shown that there is little difference between the infiltration rate from the inner cylinder and that of the larger cylinder Increasing the size of the infiltrometer is the only way to reduce the effect of lateral divergence of the flow below the cylinder.” H Bouwer, Methods of Soil Analysis, Amer Soc of Agronomy, 1986 Double Rings Infiltrometers •Outer Rings are to 24 inches in diameter (ASTM - 12 to 24 inches) •Mariotte bottles can be used to maintain Constant Head •Rings Driven - cm to inches in the soil and if necessary sealed Double Rings Infiltrometers Ring infiltrometers: other problems     How you know when to stop? Typically stop when several successive measurements show no change in the amount of water added; Reality is that you never know if the rate will change again—almost anything can happen! Takes longer to reach the asymptotic infiltration rate:  Under dry conditions;  For finer-textured soils; Typical infiltration curve: real data Effect of soil type on ring infiltration accuracy (60-cm ring, cm of ponding) Soil type Infiltration/Ks Coarse sands or gravels 1.3 Most agricultural soils 1.6 Clays and silts 2.3 Compacted clay or silt 5.7 Effect of ponding depth (60-cm ring, most soils) Ponding depth (cm) Infiltration/Ks 1.6 10 1.8 20 2.3 40 3.2 Rainfall Simulations Rainfall Simulations Rainfall simulators  Advantages of rainfall simulators:     Can control amount and rate of precipitation; Rainfall simulates actual rainfall, in contrast to a ring infiltrometer; Can select locations to vary slope, cover, soil type, or other factors; Rainfall simulators: Disadvantages   Need water supply, so usually have to the simulations close to a road; Logistically difficult to plots larger than about m2, but need larger plots to simulate other processes, such as rilling Rainfall simulators aren’t simple      Need to choose an application rate; Drops should mimic natural raindrops in size and velocity; Need to know size of natural raindrops for desired application rate; Drop size and velocity varies with rainfall rate and pressure; Very difficult to get even coverage on a plot Rainfall simulator using a jet nozzle 45N 135E 40N 130E 35N Stream Experimental plot Forest road plot Soil layer：thin Fault Soil layer：thick Out crop Bedrock Bedrock Effects of measurement methods Each methods have different sizes for target area of infiltration measurement Single ring infiltrometer Plot scale rainfall simulation Large scale Inftriltration rate at different methods Why so variable? ... đọng bồi tích) SOILS CONTENTS Soils = parent material + organics + water + air soil horizons - distinct soil layers soil profile - sequence of layers Soil horizons and profile Soil classification... flooding; More water for plant growth; Need to understand how water and soils interact ( soil physics”) for both land management and resource protection! Soil generation Weathering of bedrock material... the soil is voids? How much pore space in soils?    Surface soils in forests typically are what percent pore space? Clay soils typically have more or less porosity than sandy soils? Organic soils