RAINFALL SIMULATOR EXPERIMENT-- LAB 11
(Fall semester 2008)
The runoff data for all lab sections has been collected into an Excel spreadsheet for you to work out the calculations and plot the data for the different treatments used. Download the spreadsheet by clicking on the link below. The text following, and embedded in the spreadsheet, should help you to complete the lab and write the lab report. Please read this entire page, and as part of your lab report answer the discussion questions at the bottom of this page IN ADDITION TO those in your lab write-up.
Some comments on Methods:
The soils used were a Cecil Ap from a cotton field in Oconee Co.,GA, and a Bt/BC from a Cecil profile sampled at the in Clarke Co.. Soils were air-dried and screeened to < 5 mm, and runoff pans (20 cm x 40 cm) were packed with the soil to give a "moderate" mixture of fine soil and aggregates (up to 5 cm diameter) on the soil surface. Rainfall was applied for 1 h, with runoff volumes collected every 5 minutes. Multiple "control" soil pans (bare soil) for each soil were run over the 5 lab sections, along with several different mulch rates of pine straw spread on the soil surfaces. Only one pan of each of each of the mulch treatments was run.
Soils:
The Ap horizon was a coarse sandy loam from a cultivated field; it had very low aggregate stability and was very susceptable to crusting.
The Bt/BC horizon was a clay loam/clay texture, with few aggregates.
Mulch Rates: Pine straw was spread on the soils at the following rates:
Topsoil: 2, 4 and 6 t/a.
Subsoil: 2 and 5 t/a.
NOTE: the 2 t/a covered about 40-50% of the soil surface; 4 t/a covered abouat 90%; and 6 t/a covered 100%)
Results:
| The Cecil Ap soil used in these experiments has moderate aggregation, shown here on left packed in runoff pans before rainfall. The Bt subsoil was very poorly aggregated (right), mostly composed of fine, dis-aggregated soil. | |
| After rainfall at about 5 cm/h for 1 h, both soils were pretty severely beaten down, with few visible aggregates at the soil surface and many exposed fine rocks. | |
| Topsoil pans treated with pine straw after rainfall. Note surface coverage by pine straw; only at the 6 t/a rate was 100% of the soil surface covered by mulch. | |
| Subsoil pans after rainfall; you can see a lot of the red soil thru the 2 t rate, but at 5 t/a rate 95%+ of the soil surface is covered. | |
| Sediment load in the B horizons soils at the end of the 1 hr rainfall event: 5 t/a mulch on left, bare soil on right. The mulch seemed to have a large effect on the concentration of sediment in the runoff, even though the volume of sediment was not reduced very much. | |
Lab Write-up:
Make two graphs in Excel using the runoff data for the two soils (A and B horizons); it is NOT necessary to make graphs of infiltration (they are just the mirror image of the runoff curves). Copy and past these graphs into a word processor, and answer the questions at the end of the Lab 11 handout, making sure to address the following points:
--For the bare soils, why does the runoff increase (infiltration decrease) so rapidly over time of rainfall?
--How does mulch act to delay runoff and increase infiltration? Why is adding more mulch so much more effective in reducing runoff?
--At the highest mulch rate, there was very little runoff on the A horizon, but quite a bit on the B horizon by the end of the rainfall event; why does this occur? (Remeber that infiltration can be controlled either by crusting, or by the saturated hydraulic conductivity: what do you think the K-sat's are for these two different soils?)