water holding capacity, rock fragments, bulk density / data source documentation

[brownag]

• improve documentation of source of WHC/DB/RF data in KSSL snapshot
• clarify degree to which rock fragment correction occurs in the process of converting to whole soil basis
  • would it be worthwhile to get RFs from the NASIS pedon object corresponding to KSSL record?
• take into account different methods/prep codes
• allow exposure several different water retention / water retention difference columns via fetchKSSL(), for example "wrd_l2" for the <2mm water retention difference as opposed to the whole soil default "wrd_ws13"

[dylanbeaudette]

The water retention data in the snapshot (KSSL in general) are something of a mess.

For next time:

• wrd_l2: Water Retention Difference, 1/3 to 15 Bar, <2mm: ([w3cld] - [w15l2]) * [db_13b] / 100
• wrd_ws13: Water Retention Difference, 1/3 to 15 Bar, Whole Soil: ([w3cld] - [w15l2]) [wpl2] / 100. [db_13bw] / 100
• w3cld: water ret. @ 1/3 bar, wt %, clod (measured)
• w15l2: water ret. @ 15 bar, wt %, air dry (measured)
• wpl2: < 2mm (wt %) -- note this doesn't account for all >2 mm fragments!
• db_od: Db < 2mm, oven dry (measured)
• db_13b: Db < 2mm, @ 1/3 bar (measured)
• db_13bw: "Weight per Unit Volume, @ 1/3 Bar, Whole Soil"

This last one db_13bw is estimated using:

100 / 
(
[wpg2]/[pd_g2]+(100-[wpg2]) /
IIf(IsNull([Bulk Density and Moisture].[db_13b]),
1.45,
[Bulk Density and Moisture].[db_13b])
)

Notice that a default Db @ 1/3 bar of 1.45 is used when missing.

Most of the derived Db values can be found in the Supplementary_Tier_2 table.

Key reference: pages 203--207 Methods of Soil Analysis: 4 Physical Methods.

[dylanbeaudette]

As of 807a518abb97f86c53b040b76740a28eb06d2af4, there is now a wrd_l2 column in the horizon level data returned by fetchKSSL(). However, this column contains NA because most data in Supplementary_Tier_3 table have a prep code of "".

F***.