{"id":4604,"date":"2025-05-10T13:42:42","date_gmt":"2025-05-10T13:42:42","guid":{"rendered":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/?post_type=ht_kb&p=4604"},"modified":"2025-09-05T13:03:40","modified_gmt":"2025-09-05T13:03:40","slug":"malcom-small-watershed-hydrograph","status":"publish","type":"ht_kb","link":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/knowledge-base\/malcom-small-watershed-hydrograph\/","title":{"rendered":"Malcom Small Watershed Hydrograph"},"content":{"rendered":"\n

The Malcom method hydrograph, also known as the Small Watershed Hydrograph Method, originated from a watershed research study led by H. Rooney Malcom in 1980 in North Carolina. This research aimed to explore stormwater detention as a strategy for mitigating urban streambank erosion and flooding, focusing on identifying the optimal design storm for stormwater detention programs. <\/p>\n\n\n\n

A significant outcome of this study was the development of a runoff hydrograph method that effectively integrates the Rational method and the NRCS method, which are well-regarded by engineers. This approach is user-friendly and offers watershed modelers the advantages of both techniques, calculating peak flow using the Rational method while resembling the NRCS unit hydrograph in shape and volume. <\/p>\n\n\n\n

\n

One could say that NRCS and Rational got married, had a child… and named it Malcom!<\/p>\n<\/blockquote>\n\n\n\n

The Malcom method is gaining popularity across the country, especially in Texas, where it is mandated in certain jurisdictions for the design of detention facilities.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Malcom Method Inputs<\/h2>\n\n\n\n

Due to the meshing of the two methods, Malcom hydrograph inputs are the same as the those for the Rational method:<\/p>\n\n\n\n

Drainage Area
Runoff Coefficient, C
Time of Concentration, Tc<\/p>\n\n\n\n

Plus the NRCS Curve Number, CN.<\/p>\n\n\n\n

Hydrology Studio still allows you to compute a Composite C but only a single value for the corresponding CN. CN can be calculated as:<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Where:<\/p>\n\n\n\n

IMP = % impervious
X is taken from the table below and is based on the project’s Soil Type.<\/p>\n\n\n\n\n\n\n\t\n\n\t
Soil Type<\/th>A<\/th>B<\/th>C<\/th>D<\/th>\n<\/tr>\n<\/thead>\n
X =<\/td>39<\/td>61<\/td>74<\/td>80<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n\n\n\n

For a C = 0.50 and a Soil Type D, an equivalent CN is 98(.50) + 80(1-0.50) = 89.<\/p>\n\n\n\n

It is not mandatory but highly recommended to use a Time Interval of one minute for Malcom hydrographs. The Time Interval can be changed in\u00a0Project Settings<\/a>.<\/p>\n\n\n\n

Malcom Small Watershed Hydrograph Equations<\/h2>\n\n\n\n

The Malcom hydrograph is constructed in a way that closely resembles the shape of the NRCS Unit hydrograph. It employes two distinct equations to compute flows, Qi<\/sub>, at a given time, ti<\/sub>.<\/p>\n\n\n\n

\"Malcom
The Malcom Hydrograph combines NRCS with the Rational method.<\/figcaption><\/figure>\n\n\n\n
\"\"<\/figure>\n\n\n\n
\"\"<\/figure>\n\n\n\n
\"\"<\/figure>\n\n\n\n

Where:<\/p>\n\n\n\n

Tp<\/sub> = Time to Peak
V = Volume (cuft) = Excess precipitation, Q, of the currently selected NRCS design storm (in) x Drainage Area (ac) x 43,560\/12
Qp<\/sub> = Peak discharge as calculated by the Rational method (Qp<\/sub> = CiA)
Ti<\/sub> = Time in seconds
Qi<\/sub> = Discharge in cfs at Ti<\/sub><\/p>\n\n\n\n

Note that the total precipitation, P, is automatically pulled from the currently selected design storm. The excess precipitation, Q, is calculated from the following curve number equation:<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Where:

Q = Excess volume of precipitation (in)
P = Total precipitation (in) for the selected design storm
S = Potential maximum retention
= (1000 \/ CN) \u2013 10
CN = Curve Number (Hydrology Studio\u2019s published tables of Curve Numbers assumes an antecedent moisture condition of II)<\/p>\n\n\n\n

It is essential to recognize that this method necessitates both a Curve Number (CN) and a Runoff Coefficient (C). These values must accurately reflect their associated land use. Below is a straightforward approach to derive an equivalent CN from a Rational Method C. It is logical to conclude that the Rational Method C indicates a percentage of imperviousness; for instance, a C value of 0.60 signifies 60% impervious surface. An equivalent CN can be calculated as:<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Where:<\/p>\n\n\n\n

IMP = % impervious
X is taken from the table below and is based on the Soil Type.<\/p>\n\n\n\n\n\n\n\t\n\n\t
Soil Type<\/th>A<\/th>B<\/th>C<\/th>D<\/th>\n<\/tr>\n<\/thead>\n
X =<\/td>39<\/td>61<\/td>74<\/td>80<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n\n\n\n

For a C = 0.60 and a Soil Type C, an equivalent CN is 98(.60) + 74(1-0.60) = 88.40.<\/p>\n\n\n\n

\"Malcom
A Malcom method hydrograph combines the advantages of Rational and NRCS<\/figcaption><\/figure>\n\n\n\n

A Comparison of Malcom, Rational and NRCS<\/h2>\n\n\n\n

The following is a comparison of the Malcom, NRCS, and Rational methods. All three hydrographs utilized similar inputs, resulting in nearly identical Q peaks. However, the volume produced by the Rational method is significantly lower. Since the excess runoff in the NRCS method is non-linear, one should not anticipate Rational method matching peak discharge values across all return periods.<\/p>\n\n\n\n

Drainage area = 10 (ac)
Tc = 20 minutes
Runoff Coefficient (C)= 0.60
CN = 88 (Soil Type C)
Total Precipitation from SCS Type II 24-hour storm<\/p>\n\n\n\n

\"Malcom,
From left to right: Rational, Malcom, NRCS<\/figcaption><\/figure>\n\n\n\n\n\n\n\t\n\n\t\n\t\n\t
Hydrograph Method<\/th>Peak Flow (cfs)<\/th>Volume (cuft)<\/th>\n<\/tr>\n<\/thead>\n
Rational<\/td>40.91<\/td>49,093<\/td>\n<\/tr>\n
Malcom<\/td>40.91<\/td>131,595<\/td>\n<\/tr>\n
NRCS (SCS Type II 24hr)<\/td>41.04<\/td>131,595<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n\n\n\n

The Malcom method combines the advantages of both approaches, offering the straightforward Peak Q from the Rational method alongside the realistic volume estimates provided by the NRCS method.<\/p>\n\n\n\n

<\/p>\n","protected":false},"excerpt":{"rendered":"

The Malcom method combines the advantages of the NRCS and Rational methods<\/p>\n","protected":false},"author":1,"comment_status":"open","ping_status":"closed","template":"","format":"standard","meta":{"footnotes":""},"ht-kb-category":[24],"ht-kb-tag":[75,62,63],"class_list":["post-4604","ht_kb","type-ht_kb","status-publish","format-standard","hentry","ht_kb_category-building-your-basin-model","ht_kb_tag-malcolm","ht_kb_tag-malcom-method","ht_kb_tag-malcom-small-watershed-hydrograph-method"],"_links":{"self":[{"href":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/wp-json\/wp\/v2\/ht-kb\/4604","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/wp-json\/wp\/v2\/ht-kb"}],"about":[{"href":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/wp-json\/wp\/v2\/types\/ht_kb"}],"author":[{"embeddable":true,"href":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/wp-json\/wp\/v2\/comments?post=4604"}],"version-history":[{"count":23,"href":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/wp-json\/wp\/v2\/ht-kb\/4604\/revisions"}],"predecessor-version":[{"id":4673,"href":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/wp-json\/wp\/v2\/ht-kb\/4604\/revisions\/4673"}],"wp:attachment":[{"href":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/wp-json\/wp\/v2\/media?parent=4604"}],"wp:term":[{"taxonomy":"ht_kb_category","embeddable":true,"href":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/wp-json\/wp\/v2\/ht-kb-category?post=4604"},{"taxonomy":"ht_kb_tag","embeddable":true,"href":"https:\/\/learn.hydrologystudio.com\/hydrology-studio\/wp-json\/wp\/v2\/ht-kb-tag?post=4604"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}