Take home exam soil 456 Spring 2008


  1. A soil sample was collect that had 1 inch of sand 0.5 inches of clay an 0.5 inches of silt measured in a jar. What is the soil type and what are the bulk density, field capacity, permanent wilting point and soil water hold capacity of the soil based on Saxtonís model. For 5 points extra credits explain how the sexton calculator works. Be specific about any equation used in the calculator. Points 10


%sand= Ĺ=50% on a volume =cm^3 sand/cm^3sand+cm^3silt +cm^3clay

%silt= 0.5/2=25% on a volume

%clay= 25% on a volume††††

These are the values to put into sexton calculator assuming bd = 1

To convert to weight bases (g of sand/g sand+g silt +g clay) you must know the bd of the sand silt and clayIf you used the bd listed in the table then you would be assuming that the material settled to density found in soil sample which would probably be too high but using those number then convert cm^3/cm^3 to g/g by assuming 1 cm^3 of material equal to .5 cm^3 of sand .25 cm^3 of silt and .25^cm clay convert to g = . 81 cm of sand .315 g of silt and . 287 g of clay add total wt = 1.412 g and divide into each to equal to 22 % silt 20 % clay and 58% sand.

Soil type sandy clay loam assuming bd=1


Fc 26

Pwp =15

            in/ft aw

soil type sand loam

fc 24

pwp 13

whc= 1.25


Saxton model develop water content curve vswater potential based on a power function curve with a and be coefficient determined by a regression model of a= function of percent sand , silt claytaken in different powers and combinations and the same for b. The is used from water potential from 10 to 1500kpa . Saturation to air entry air entry the water potential is constant at air entry. A liner relationship with an a and b coefficient is defined with the independent variable water content and the dependent variable water potential. The saturation content is a liner function of sand and clay. Between air entry and 10 kpa a liner interpolation is used.


The water content for field capacity is assumed to be at 10 bar or 100 kp of suction and perminate wilting point is at 15 bars or 1500 kpa.


  1. The city of Alamogordo gets its water from a Bonato Lake in the mountains with the surface of the lake located at 7380ft. The bottom of the lake is at 7320 ft. The distance from the lake to Alamogordo is 70 miles and the friction loss through the pipe is 2674 ft. The flow rate of the pipe is 38 cfs and the pipe size in old steel having an inside diameter of 3 ft and a Hazen Williams coefficient of 70. A pipe runs from the bottom of the lake to the city distribution site at 4220 ft. What is the water pressure at the distribution site? If you have a house that is located a mile away at an elevation of 4500ft and the friction loss in the city distribution system is ignored what is the pressure at your water meter.Points 15. For an extra 5 points what is the velocity in the pipe from the lake to Alamogordo. Show work or capture screen if you are using an internet calculator.

    Answer 5.37 ft/ sec velocity and 206 ft or 89 psi at house and 486 ft or 210 psi at the distribution point.

  2. A pecanfield is flood irrigated with a high flow turnout that discharged 3375 gpm into the field. The boarder in the field is 300 ft long and 120 ft wide. The irrigation-scheduling model determined that a irrigation was due on May 20 and a 3.8 inch irrigation was need to fill the root zone which would result in 3.0 inches of water being stored in the root zone. The farmer tells the irrigator to go and turn water into the field until the water reaches 75% of the way to the end of the field and then shut the water off. The irrigator does this and reports to the farmer that it took 30 minutes to irrigate that boarder. The field contains 6 boarders and the irrigator was through irrigating after 3 hr.

    What was the depth of water the irrigator applied to the field? What was the target efficiency and what was the final irrigation efficiency that the farmer calculated that occurred using the rule of thumb to shut the water off when it reaches ĺ of the length of the field and let it cost the rest of the way. Point 15

    Answer qt=ad q=7.5 t=0.5 a=.82 ac d= 4.5 inches target efficiency 3/3.8 =78.9% actual 3/4.5= 66%

  3. Explain the different type of irrigation scheduling methods and the advantages and disadvantages of each and which method you would use to schedule irrigation for a landscape around a home. Explain why you choice that method. You can automate the irrigation scheduling methodat the house as your method of choice but you have to explain how the equipment would work. Points. 20


Soil moisture monitoring advantage Ėdisadvantage

Water balance

Condition of crop

Infrared temperature method


Select the easiest looking at condition of crop.




  1. Winter wheat is grown on the high plains in Texas and in New Mexico. Clovis in near the Texas. Using the climate data from Clovis calculate both the dry land yield and fully irrigated yield for the harvest years 1992 and 2000 and the growing season ET. Wheat is planted in September 1 and harvested July 10 of the next year. Maximum wheat yield under full irrigation is 80 bushels/ac.Points 20


60 lb = 1 bushel so 80 bushels= 4800 lb/ac



Non stressed yield is 4800 lb/ acEt 1992=28.47†††††††††††††††††† Et 2000= 42.2 inches


Dry land†† 1992 et= 11.5 yield = 134 lb/ ac†††† 2000 et= 13.6 yield= -268=0

lb/ ac


Look at the Etr generated on the web site and see that even with missing climate data a reasonable value is there because the program using samaniís equation to estimate PET as long as the temperature data is good. The water balance model only need a reasonable estimate of temperature and Eto to run. Temperature for gdd calculation. Also rainfall needs to be correct. Rainfall is at the location of the weather station and may not be good. So the dry land yield may be in error. You will notice that the kt factor in Samaniís equation defaults to 0.16 but the note says that it should be set up 0.22 for clovis so the large missing data in 1992 that then uses Samaniís equation under estimate Eto and Et. Need to run again with kt set to 0.22 if you do this then Et under non stress is 36.9 inches closer to the Et in 2000 which has less missing or bad data.

The error in eto does not matter as muchunder dry land because the et is controlled by the amount rainfall more than the potential Et rate.


Spreadsheet 1

Spreadsheet 2

Spreadsheet 3


  1. Explain what controls the infiltration process and why the infiltration rate starts out high and the decrease to the final infiltration rate close to the hydraulic conductivity of the soil. Explain what impact water high in sodium might have on the infiltration rate and why?Points 10


  1. The feel method is used to determine the soil type and the moisture content for scheduling when to irrigate. A technician went to the Pecan field at Stalmanís farm located 5 miles south of Las Cruces and reported to you that he found the following information.

††††† Sample depth†† 1ft†††† soil stays in a ball when squeezed and makes a medium ribbon and does not feel gritty answer= sand clay loam or a silty clay loam

††††† Sample depth††† 2 ft††† soil stays in a ball when squeezed and does not form a ribbon. The second foot shows subsurface horizon development with a gypsum layer answer loam sand

††††† Sample depth 3 ft††††† soil does not stays in a ball when squeezed and does not form a ribbon. Sand


What are the soil textures with depth and what is the total available water holding capacity of the 3 ft root zone. Use table 6.3 in your book for the water-holding capacity of different textured soils. Show all calculations. Also what is the soil order?Points 10


1ftsandy clay loam to a silt clay loam maybe clay loam ††1.5- 1.75 inch/ft

2ft loam sand have way between sandy loam and loam 1.75 inch/ft

3ft sand 1inch ft


Soil type aridisol