Embedded Files
Exercise objective:
· Understanding the effect of soil properties on crop performance in water-limited production situations.
Suggested reading:
The theory of water-limited production as explained in Chapter 4; the soil data file as explained in Chapter 7.4 of the book ORYZA2000: modelling lowland rice.
This set of exercises continues the treatment of the simulation of crop growth and development with a water balance. The previous exercises in “Chapter IV: Water-limited production: crop and management” focused on the crop and water management as defined in the experimental data file. Here, we concentrate on the effect of soil properties as defined in the soil data file. The soil data file we discuss contains soil characteristics as required for the soil water balance PADDY, which is the default water balance module for ORYZA2000. We continue to simulate the growth and development of rice variety IR72, in the dry season of 1992, using weather data from the IRRI farm in Los Baños, the Philippines.
PADDY simulates the water balance of lowland rice soils. Usually, a lowland soil is puddled: repeated plowing and harrowing under flooded conditions to create a 15-20 cm layer of mud. Under the mud, a 2-5 cm compact plow sole is present that ‘blocks’ the downward flow of water from the flooded muddy layer above. Under the plow sole, there is the unpuddled and undisturbed subsoil. Like most water balance models, PADDY assumes that the soil is divided in a number of layers. For each layer, PADDY dynamically simulates water flow (upward and downward), soil water content and soil water tension. For that purpose, for each layer the so-called hydraulic conductivity and water retention characteristics are required. The ‘conductivity characteristic’ represents the relation between the conductivity for water movement and soil water content, and the ‘water retention characteristic’, the relationship between soil water content and soil water tension (or pF curve). Different functions and equations have been developed to describe these relationships. One of the most well-known are the ‘Van Genuchten’ equations that describe both relationships with one set of (6) parameters: saturated conductivity, saturated volumetric soil water content, residual volumetric soil water content, and three empirical parameters (a, l, n). The derivation of these parameters (for each soil layer!) from hydrological measurements is cumbersome, but the use of Van Genuchten parameters in water balance models generally gives accurate results. PADDY accepts two alternatives to the Van Genuchten parameters: measured water retention data (volumetric soil water content at saturation, field capacity, wilting point and air-dryness) for the water retention characteristic, and a so-called ‘power function parameter’ for the conductivity characteristic. For non-puddled soils, water retention and conductivity characteristics have often been measured, published and made available. Pedotransfer functions have been developed that relate these characteristics to other soil properties that are more easily measured, such as soil texture, bulk density and soil organic matter content. All this information can be used to derive soil water retention and conductivity parameters for the non-puddled subsoil of lowland soils. However, the puddled topsoils of paddy fields have hardly been measured and hydrological properties for the muddy layer or the plow sole are scarce. Therefore, PADDY has as ‘short cut’ option to simulate water flow in the puddled topsoil. Instead of the conductivity characteristics, we only have to supply a general daily percolation rate. This characteristic is more easily measured and is more widely available than conductivity characteristics. A further typical feature of PADDY is that it can simulate the effect of cracking of the puddled soil under continued drying. When a certain threshold (soil water content) level is reached, cracks can penetrate the plow sole and completely change the conductivity characteristics. For cracking soil types, this threshold level can be supplied as a maximum soil water tension. Finally, PADDY requires some general input data such as height of the bunds, depth of the groundwater table, and initial conditions (depth of ponded water and soil water content).
PADDY can also simulate the water balance of a non-puddled soil. However, it is still assumed that it is a typical lowland paddy soil, which means that the topsoil is rather impermeable and that there is usually ponded water on the surface. For non-puddled soils, PADDY requires the (empirical) daily percolation rate of the topsoil. For more aerobic or upland soil conditions, PADDY is less suitable and an alternative soil water balance, such as SAWAH should be used (SAWAH is incorporated in the most recent version of ORYZA2000, but the treatment of this model is beyond the scope of these exercises).
Ex-V.1. Start up the Shell FSEWinRunOnly. Open project C:\COURSE\SOIL\ORYZAWin. Verify the contents of CONTROL.DAT:
FILEON = 'RES.DAT' ! Output file
FILEOL = 'MODEL.LOG' ! Log file
FILEIT = 'C:\COURSE\SOIL\EXPLORE.DAT' ! Experimental data
FILEI1 = 'C:\COURSE\SOIL\IR72.D92' ! Crop data
FILEIR = 'C:\COURSE\SOIL\RERUNS.DAT' ! Rerun file
FILEI2 = 'C:\COURSE\SOIL\PADDY.DAT' ! Soil data
Study the soil data file PADDY.DAT and try to identify the information described above.
Our example data file is for a puddled soil that is divided into nine soil layers (NL=9). The first six layers are each 5 cm thick, the next layer is 10 cm, and the last two layers are each 20 cm thick. Note the flexible way in which the array TKL is filled! The puddled topsoil consists of three layers (NLPUD=3), and for each layer we have to supply the saturated water content after drying (after the ‘mud’ has turned into ‘soil’). We supply groundwater data, with a groundwater table at 150 cm depth throughout the year. The percolation rate of the puddled topsoil is 3 mm per day. We use Van Genuchten parameters to characterize the water retention and conductivity properties for each of the nine soil layers. Note that, since we chose to use Van Genuchten parameters, we do not need to supply the power function parameter nor the water retention data (these values are not activated since they are preceded by an asterisk (*)). In this example, we initialize the water balance with a layer of 10 mm ponded water and all soil layers saturated with water. At transplanting, we re-initialize the water balance, since we only keep track of the water balance of the main field. If we simulate a direct-seeded crop, we do not need to re-initialize the water balance.
Percolation: Irrigated conditions
Exercise objective:
· Effect of percolation rate and plow sole conductivity; irrigated conditions.
Suggested reading:
Chapter 7.4 of the book, ORYZA2000: modelling lowland rice.
Click HERE for learning exercises on Percolation: Irrigated conditions.
Bund height: Rainfed conditions
Exercise objective:
· Effect of bund height; rainfed conditions.
Suggested reading:
Chapter 7.4 of the book, ORYZA2000: modelling lowland rice.
Click HERE for learning exercises on Bund height: Rainfed conditions.
Groundwater table depth: Rainfed conditions
Exercise objective:
· Effect of groundwater; rainfed conditions.
Suggested reading:
Chapter 7.4 of the book, ORYZA2000: modelling lowland rice.
Click HERE for learning exercises on Groundwater table depth: Rainfed conditions.
Water retention data: Rainfed conditions
Exercise objective:
· Use of measured water retention characteristics; rainfed conditions.
Suggested reading:
Chapter 7.4 of the book, ORYZA2000: modelling lowland rice.
Click HERE for learning exercises on Water retention data: Rainfed conditions.
Experiment simulation
Exercise objective:
· Simulating an irrigated/rainfed experiment with observations
Click HERE for learning exercises on Experiment simulation.
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